In Universe, reason is the fact to many cons, Megatron is offically dead, its likely been spread by Starscream and the other surivors of the assult on Autobot city, when Galvatron & Cyclonus unexpectily showed up at Starscream's coronation and killed him, Rumble asked "What was his name again?" And he proclaimed himself, Galvatron, it was likely the higher ups present in the throne room that day and the most low level Decepticons were likely listening or watching a boardcast from elsewhere on Cybertron, after all the Decepticons control Cybertron and it takes more than just a handful of bots to maintain control over the whole planet, so when the reformated Megatron proclaimed himself, Galvatron, it was heard across the planet and Galvatron let the Autobots know who was attacking them with the sudden/direct attack on Autobot city, thus his enemies know who he is, so, he chosen to keep the name to go with his new body, much like the cons who were reformated into Cyclonus & Scourge kept their names, the Sweeps its questionable thou they more or less became drones under Scrouge's command, while Armada got roasted by Grimlock's firebreath when the Autobots fled Autobot city that day, its also like Hot Rod being given the name Rodimus Prime to go with his evolved look, but the first real known change of body and idenity would be Orion Pax to Optimus prime, we believe Aerial became Elita One, while everyone speculates that Dion became Ultra Magnus, so idenity changes are all part of Transformers lore/history, so why question it?
