I mean yeah, it's newer, but there should be a considerable reason to stop using VX ACE while you're that far into it, and it's not like MV is that much more advanced in functionality, majority of what you might want to do in your rpg you should be able to make without MV additions. Did he ran out of variables and switches, or something? The only reason i can see in doing it in the middle of your project is mobile version, and that. Or is it that much easier to use, and will drastically speed up the development? It would be interesting to know why exactly is he doing it, if it's not just a scam for milking money longer while doing nothing, of course.