The whole point of the seed analogy in the Pauline epistle of Corinthians cannot be separated from his discourse in Romans. What happens when a seed dies? It brings forth fruit:


John 12:24-25 Verily, verily, I say unto you, Except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit. 25 He that loveth his life shall lose it; and he that hateth his life in this age (Jewish age) shall keep it unto life eternal (the everlasting NC age or "the age to come [Luke 20]).
1 Corinthians 15:36 Thou fool, that which thou sowest is not quickened, except it die:
Romans 6:3-11 Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death? {4} Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life. {5} For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection: {6} Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin. {7} For he that is dead is freed from sin. {8} Now if we be dead with Christ, we believe that we shall also live with him: {9} Knowing that Christ being raised from the dead dieth no more; death hath no more dominion over him. {10} For in that he died, he died unto sin once: but in that he liveth, he liveth unto God. {11} Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord.
The early first century believers were dying to sin and the law and had life working in them. But they were all connected to a greater tree that was growing: a New covenant tree that was filled with Jews and Gentiles. They were falling of the tree of sin and death under the law (hence, dying to the law of sin and death) and being grafted into the new tree. That is Paul's whole point in Romans eleven:
Romans 11:15-16 For if the casting away of them be the reconciling of the world, what shall the receiving of them be, but life from the dead? {16} For if the firstfruit be holy, the lump is also holy: and if the root be holy, so are the branches.
It is all about fruit and branches and trees and seeds etc.

Notice the fruit here as a result of "death" to the law of sin:

Romans 7:5 For when we were in the flesh, the motions of sins, which were by the law, did work in our members to bring forth fruit unto death.
While alive to the law of sin and death all their works were fruit unto death. But because they had died and began the transforming work of the Spirit, which was taking place during the first century (Gal 3:3 Are ye so foolish? having begun in the Spirit, are ye now made perfect by the flesh? and...Philippians 1:6 Being confident of this very thing, that he which hath begun a good work in you will perform it until the day of Jesus Christ:), they were now bringing forth fruit unto everlasting life:
Romans 6:22 But now being made free from sin, and become servants to God, ye have your fruit unto holiness, and the end everlasting life.
Of course the question arises from vv. 6 and 8:
Romans 6:5 For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection:
Romans 6:8 Now if we be dead with Christ, we believe that we shall also live with him:
How come Paul uses the future tense?

I have addressed this issue before, but I would like to address it again:

There is only one everlasting life. There is only one image of Christ. Paul says those believers were being changed into that image:

NRSV 2 Cor 3:18 And all of us, with unveiled faces, seeing the glory of the Lord as though reflected in a mirror, are being transformed into the same image from one degree of glory to another; for this comes from the Lord, the Spirit.
Virtually every translation supports the present tense action taking place:
Young's Literal: "and we all, with unvailed face, the glory of the Lord beholding in a mirror, to the same image are being transformed, from glory to glory, even as by the Spirit of the Lord. "
NASB "But we all, with unveiled face, beholding as in a mirror the glory of the Lord, are being transformed into the same image from glory to glory, just as from the Lord, the Spirit. "
RSV And we all, with unveiled face, beholding the glory of the Lord, are being changed into his likeness from one degree of glory to another; for this comes from the Lord who is the Spirit."
NIV "And we, who with unveiled faces all reflect [1] the Lord's glory, are being transformed into his likeness with ever-increasing glory, which comes from the Lord, who is the Spirit."
The point I am making is this:

How many images are there of Christ? I hope we all would agree there is one. Paul says they were being transformed into that very image. So if there is one, it is that same image of which Paul speaks here:

1 Corinthians 15:49 And as we have borne the image of the earthy, we shall also bear the image of the heavenly.
It is the same image of Christ. The whole context supports that:
1 Corinthians 15:47-49 The first man is of the earth, earthy: the second man is the Lord from heaven. {48} As is the earthy, such are they also that are earthy: and as is the heavenly, such are they also that are heavenly. {49} And as we have borne the image of the earthy, we shall also bear the image of the heavenly.
We have to understand that Paul's use of earth and heaven is covenantal through and through:

Earth=OC - Heaven=New Covenant. It is not dealing with physical existence. It is dealing with covenantal existence. Otherwise Paul would have had no purpose whatsoever in saying this:

1 Corinthians 15:56 The sting of death is sin; and the strength of sin is the law.
Look at the context:
1 Corinthians 15:54-56 So when this corruptible shall have put on incorruption, and this mortal shall have put on immortality, then shall be brought to pass the saying that is written, Death is swallowed up in victory. {55} O death, where is thy sting? O grave, where is thy victory? {56} The sting of death is sin; and the strength of sin is the law.
The reason Paul uses the future tense for image in 1 Corinthians 15:49 and the present tense for being changed into His image in 2 Corinthians 3:18 is solved when we grasp the transformation. They were being changed. They were not completely changed yet (change=brought completely into the holiest of all); they were entering into the veil-check the tense in Hebrews 4:3
Hebrews 4:3 For we who have believed enter (present tense) that rest, just as God has said, "As in my anger I swore, 'They shall not enter my rest,'" though his works were finished at the foundation of the world.
To prove that they had not completely entered that rest, the writer of Hebrews says:
Hebrews 4:9-11 There remaineth therefore a rest to the people of God. {10} For he that is entered into his rest, he also hath ceased from his own works, as God did from his. {11} Let us labour therefore to enter into that rest, lest any man fall after the same example of unbelief.
They were entering in together. As they were entering into that holiest of all, they were gradually seeing His glory or "beholding" His glory more and more. Once the Parousia (presence) took place (the destruction of the Temple), they were immediately beholding His glory completely (not physically, audibly, visually, tangibly, but covenantally, relationally, spiritually, positionally-all of the "allys" that there are to show the blessings of Christ upon the one who has faith in Him). Hebrews proves this:
Hebrews 9:8 By this the Holy Spirit indicates that the way into the sanctuary has not yet been disclosed as long as the first tent is still standing.
But soon it would happen:
Hebrews 10:19 Having therefore, brethren, boldness to enter into the holiest by the blood of Jesus,
Colossians 3:4 When Christ, who is our life, shall appear, then shall ye also appear with him in glory.
This was an answer to Christ's High Priestly prayer (which Hebrews is addressing):
John 17:24 Father, I will that they also, whom thou hast given me, be with me where I am; that they may behold my glory, which thou hast given me: for thou lovedst me before the foundation of the world.
Friends, there is only one glory, one presence, one covenantal relationship, one spiritual reality, and either it is completely fulfilled or it is not. The only ones who experienced the gradual process were those in the first century who were growing up into Him as the body of Christ. That mustard seed was growing into a tree, into the fullness of Him who was filling all in all:
Ephesians 1:22-23 And hath put all things under his feet, and gave him to be the head over all things to the church, {23} Which is his body, the fulness of him that filleth all in all.
Back to seed.

The Greek word Paul uses is sperma:

1 Corinthians 15:37-38 And that which thou sowest, thou sowest not that body that shall be, but bare grain, it may chance of wheat, or of some other grain: {38} But God giveth it a body as it hath pleased him, and to every seed his own body.
4690. sperma, sper'-mah; from G4687; something sown, i.e. seed (includ. the male "sperm"); by impl. offspring; spec. a remnant (fig. as if kept over for planting):--issue, seed.

Notice the ways in which sowing and sperma are used:

Matthew 13:37 He answered and said unto them, He that soweth the good seed is the Son of man;

Mark 4:31-32 It is like a grain of mustard seed, which, when it is sown in the earth, is less than all the seeds that be in the earth: {32} But when it is sown, it groweth up, and becometh greater than all herbs, and shooteth out great branches; so that the fowls of the air may lodge under the shadow of it.

Even in Mary's monumental Magnificat she declares:
Luke 1:54-55 He hath holpen his servant Israel, in remembrance of his mercy; {55} As he spake to our fathers, to Abraham, and to his seed for ever.
Paul strongly addresses this spermatic relationship to Abraham:
Galatians 3:16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
The Seed is Christ, and there were many seeds falling off the Jewish OC tree and being joined to the singular Seed of Christ through faith. They were individuals who were dead to Christ and alive to sin, united with that body of OC death under the law. But in Christ they were no longer male, female, bond, free, Jew, Gentile, but rather they were now all one in Christ, united together and raised together in and with Him.
Galatians 3:19 Wherefore then serveth the law? It was added because of transgressions, till the seed should come to whom the promise was made; and it was ordained by angels in the hand of a mediator.
Finally, in terms of sowing:
Galatians 6:8 For he that soweth to his flesh shall of the flesh reap corruption; but he that soweth to the Spirit shall of the Spirit reap life everlasting.
Those under the law were still sowing to the flesh. Those in the Spirit were sowing everlasting life.

That is precisely what we have now in Christ.