I suppose if transracialism is acceptable he might not be the first black president if he declares that actually he is white.
He might also be the first feline president if transspecism is acceptable.
Well. That's another rule. If you change your species later on, then everything you did is retroactively attributed to your new species-- it's the same rules as having a gender change.
So, for example, suppose Obama becomes a female cat before he dies. If he does this before he dies, then Obama could still be the first black, female, feline, president.
I hope that clarifies things.