how can you introduce transsexualism in a gender specific magic system?

[Bohandas]

You could still argue that going from one or the other to intersex or hermaphoraditic, or going from intersex or hermaphoraditic to one of the other is still trans.

One could, for example, make the argument that Earthworm Jim is trans, since he identifies exclusively as a he, despite the fact that he is an earthworm, and earthworms are hermaphoraditic, and he still has visible female earthworm characteristics (that band thing is part of his egg-laying apparatus)