G1 Megatron was smart; he engineered and constructed the Stunticons. Galvatron seems unintelligent and simple-minded, to put it kindly. How can they be the same guy.
I did not see Transformers: The Animated Movie in the 80's. I just went straight to watching season 3 episode here and there. I only watched it once in a while because I was turned off by the drastic change. I missed the episode or scene where Astrotrain referenced what happened in the movie. All I knew was that Optimus Prime died and that's why he was not there. I asked my friends at school what happened to Megatron because I did not make the connection when I saw Galvatron in the season 3 cartoon that he was Megatron. I was wondering where he came from. I asked my friends at school if Megatron died and they said yes; they did not tell me that Galvatron came from Megatron. I did not ask about Galvatron because I was not really interested in him.
I only later on watched the animated movie to find out how Optimus Prime and Megatron died, and where Rodimus Prime and Galvatron came from. Well I guess you can imagine my surprise to find out Galvatron originated from Megatron.
The point is I was was watching season 3 objectively, without knowledge of the animated movie, and I did not make the connection between Galvatron being Megatron. And I think if you present those two characters to someone with no knowledge of Transformers they would assume they were different characters.
Didn't Teletraan One reformat all the Autobots and Deceptions after they crash-landed on earth? Yet they kept all their same names and identities.
I don't like when fans say Megatron was just reformatted. What Teletraan 1 did was reformat. According to the Teletraan 2 files, "Megatron was reformed into a new robot named Galvatron." Fans just say reformat because it sounds cooler and more techy.