Not the body which shall be (ou to sōma to genēsomenon). Articular future participle of ginomai, literally, “not the body that will become.” The new body (sōma) is not yet in existence, but only the seed (kokkos, grain, old word, as in Mat 13:31).
It may chance (ei tuchoi). Fourth class condition as in 1Co 14:10 which see. Paul is rich in metaphors here, though usually not so (Howson, Metaphors of St. Paul). Paul was a city man. We sow seeds, not plants (bodies). The butterfly comes out of the dying worm.