I mean, for all the problems with these movies, that's something that holds up pretty well..
Galvatron was the name of the Transformer created by the humans using Megatron's remains. Megatron's consciousness took over, so why would he not go back to calling himself Megatron? Also, I don't remember if he ever called himself Galvatron in AoE.. Even if he did though, given time, is it that big a deal if he went back to his first name (apart from obvious fan reasons -- but having humans make Galvatron is also pretty bad, soooo.....).