Shia can out act anyone his age, and quite a few in the next decade bracket, many of which make much more money than he does... and Voight personally mentors him, so he's guaranteed to be better than last time you saw him...
As for voices, I say keep Peter Cullen, cuz he can still pull it off and Optimus Prime is iconic in many ways... I would even say keep Chris Latta as Starscream... the rest of the voice actors have been out of work for a while, likely they had better things to do... I'm not sure about Frank Welker, while clearly the most experienced, I'm not sure his voice does it for a darker more adult Megatron... but hey, what the heck...
All the other vehicles should be replaceable, imho... of course, Tim Curry makes for a good replacement for Megs, since Welker's a bit older now... James Earl Jones would make an awesome Ironhide, whlie I'd love to hear Pearlman as Prowl. Dimaggio would be the ULTIMATE Starscream, and, well... yeah... I'd like to see some good things in the future...