^^ Er, no; Power Girl is an alternate version
of Kara/Supergirl, who has AFAIK always existed alongside Superman in some form, whereas I'm proposing replacing Superman for this particular movie. An entirely different kettle of fish. As for the alternate universe angle, so long as there weren't any crossovers to Kal-El's universe, I hardly think audiences would be confused, seeing as how they embraced the loopier Trek XI
without any trouble. It'd just be a Superman story, maybe showing Ma and Pa Kent, but with a chick. No problemo.
Speaking of which, I always wanted to see a Smallville
episode with this sort of conceit, in which Clark somehow enters an alternative reality where a daughter of Jor-El and Lara came to Earth. Clark and his "sister" could then kick some butt together before the former went home.