I'll be honest: A lot of cartoon characters go through
something vocal evolution. Even though the entire TYT/MYM voice cast sounded exactly the same as the ANG ones from the start, there's a good chance they would sound different by the end of G5. I mean, in G4, Applejack and Nightmare Moon in later seasons sounded very different from their season 1 selves. I'm actually happy that some of the G5 characters sound different in TYT and MYM, because it already makes the period of weirdness with the new voices happen sooner and end faster.