That makes me wonder, would trans-racialism become a thing if genetic modification for alteration of melanin levels became possible?
No; no more than it has become a thing because of plastic surgery, bleaching, and all other manner of body-altering shit that's come about.
Transgender exists because every human being has the potential capability of displaying a multitude of physiological gender and sexual signifiers regardless of birth, and sometimes they don't always match.
Race is nothing but the arbitrary social classifications to discern between people with particular phenotypical and ancestral traits, of which humans do
not have said potential capability of displaying regardless of birth because
these things can only be directly passed down in the family line.
Take dogs.
Any breed of dog can be male and female or have a mix of gender traits. However, a purebred dalmatian can't suddenly become a damn purebred corgi, even if it had the sapience to express the desire to really, really, really wanna be one.