Paul is specific about this--the very body that was buried ("sown") in death is raised. That body is the "it" that is bolded numerous times in the passage above. There's nothing else available for an antecedent. It is raised better than it was before, but it is still raised. It is raised "imperishable", meaning it is meant to last forever. Thus, when Jesus Christ was raised, never to die again, it was His flesh that was raised, not His Spirit.
The key to understanding the verses which you quoted is found earlier:
"Thou fool, that which thou sowest is not quickened, except it die: 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" (1 Cor.15:36-37).
Here Paul compares the natural body to a "seed." What is sowed or planted is the seed, the natural body. And until the seed dies and is rotted in the ground it does not produce anything. But once it dies then it brings forth the "body which shall be."
After the physical body is destroyed something springs out of it and it is widely different from the original body (seed) and the form is completely different. It is ridiculous to argue that the splendor of a beautiful flower is in the same form and splendor as the "seed" from whence it came.
With that in mind let us look at these verses:
"It is sown in corruption; it is raised in incorruption: It is sown in dishonour; it is raised in glory: it is sown in weakness; it is raised in power: It is sown a natural body; it is raised a spiritual body" (1 Cor.15:42-44).
The natural body is compared to a "seed" and that seed is planted in corruptible form and then it dies in the ground. What grows out of that seed is completely different from the seed because it grows into something that is not corruptible. When the natural body dies in the ground then what springs from it is completely different from a natural body and it becomes a "spiritual" body.
Despite all of this you say;
Paul is specific about this--the very body that was buried ("sown") in death is raised.
So we are supposed to believe that when the seed (the natural body) dies then what springs from that seed is just another seed?
No, the Apostle Paul uses an adjective to describe the body in which we will be raised and he describes our future body as a "spiritual" body. And as I said, the Greek word translated "spiritual" means: "always connotes the ideas of invisibility and of power" (Vine's Expository Dictionary of new Testament Words).
So we can understand that when Paul used that adjective to refer to our future bodies (being a "spiritual" body) he was referring to a body which is invisible to us in our natural bodies.
Now to clear up something. The Lord was raised in an earthly body. The saints who will be raised from the dead in the last day will also be raised in earthly bodies. The body in which one is raised will be suitable for the environment in which that person will exist. After all, it would make no sense for us to be raised in an "earthly" body if our destination is heaven, would it? And it would make no sense to argue that the Lord Jesus is now in heaven in an "earthly" body instead of a "heavenly" one.
I say that all will be raised in a glorious body, one which is adaptable to the environment in which it will exist. When we return to the earth with the Lord Jesus our bodies will adapt to an earthly environment. When we return to the heavenly kingdom our bodies will be adapted to that environment. Why should we imagine that our glorious bodies are not capable of such a thing.
It seems that the flesh and blood body in which the Lord Jesus was raised on the earth had some supernatural qualities which our unresurrected bodies do not possess.