Techinally, Megatron is dead in the movie and he knows that he is now Galvatron, thus the line "I'll crush you, just as Megatron crushed prime!" Of course the Autobots only presume Megatron is dead as Magnus delievers the line of " Then you'll die trying just like Megatron!"
How did they go back the other way?
Galvatron dies:
Super Megatron gets reborn: