This means that the OLD MEWTWO doesn't get the transformation, doesn't have it's voice changed, and doesn't reappear again. They don't even mention it in the movie at all afaik.
This also means the writers trolled everyone's nostalgia thinking that it will be OldTwo vs Genesect
First of all, where did you get this information?
And secondly, are they really going to bother creating a brand new Mewtwo, given its current anime-only back-story (AKA, it being unique, being developped by Team Rocket, and its original creators being kind of dead)?
What a cop-out...
"Many of you feel bad for this lamp...That is because you're crazy. It has no feelings! And the new one is much better." (IKEA's Lamp commercial)
Highly disappointed, since I would have liked a little nod to the older crowd who remembers the first movie and the Mewtwo Returns special. Serves me right for having expectations for it in the first place. =P