That's perplexingly circular. Megatron named himself after Megatron. He took the name wholesale, it wasn't changed or anything, and it was taken from the previously established Megatron, explaining why there are two people with the same exact name. G1 Megatron came first fictionally and in real life. "Megatronus" sounds fucking stupid and is a pointless since it was made-up after the fact to give Megatron a Prime-sounding name to leech off of, because he can't just be called Megatron even though he's the only one.