This passage begins and ends by stressing the differences within kinds of bodies. [source][source][source]
"(Pet lovers take note: Paul did not teach here that animals will be resurrected. He only used them as an example.)" [1][source]
The second and fifth sentences stress the differences within genus while contrasting the earthly with the heavenly. The central elements state the realities of earthly and heavenly "bodies." Structurally the passage is a chiasm. [2][source]
ANot all flesh is the same (i.e, earthly bodies).BExamples of different kinds of flesh: people, animals, birds, fishCThere are heavenly and earthly kinds of bodies.C"The splendor of heavenly bodies is of one kind and the splendor of earthly bodies is of another kind.B"Examples of different kinds of splendor: sun, moon, starsA"Not all stars (i.e, heavenly bodies) have the same splendor. [source][source][source]
In 1 Corinthians 15:39 Paul used animal life to point out the different types (substance) of flesh: human, land animals, birds, and fish. This anticipates what he said later about the earthly and heavenly existence of believers. A body can be genuinely fleshly and still subsist in different forms for different environments. The fact that there are different kinds of bodies among animals should help us understand that there can also be different kinds of human bodies. Some human bodies are mortal and some are immortal. Some are corruptible and others incorruptible. [source][source][source]
Likewise the fact that celestial bodies differ in glory (brightness) should help us realize that human bodies can also differ in glory. The glory of a perishable mortal human body is much less than that of an imperishable immortal human body. Also the differing glory of the heavenly bodies argues for differences among glorified believers. [source][source][source]