Yeah from that trailer it looks like Prime's G1 form doesn't last long, he's back to his normal look in that Grimlock fight, which is a shame as it would have been a good setup for a reboot. Yep just saw on the wiki for the film he turns into another semi style later in the film. I think they were trying to win over g1 fans. Oddly enough Galvatron in this movie sounds alot like G1 Motormaster from the stunticons.
I can't wait to see if they butchered Unicron. But i'm going to end up watching it at some point just to see Dinobots and g1 prime. I'm actually amazed the dinobots even look like dinosaurs as bad as the designs for the robots are. let's hope they get some g1 actors other than Peter Cullen for the voices.
On another note they're using the last game in the Fall of Cybertron series to tie the games with the movies which seems kind of forced to me.