M. Bison > Vega
Balrog > M. Bison
Vega > Balrog
Sagat > Sagat
Sagat was never changed. The reason for this is because he was already in the original Street Fighter I, so they couldn't change his name.

Anyway, it all started with the boxer, Mike Bison. His story and background (and obviously his name) was very similar to Mike Tyson. So to avoid legal issues when porting the game to the US, Capcom switched the names of the bosses around. Sagat remained the same due to the reason above.
But they also changed Gouki to Akuma. Not sure why, but I think that has more to do with how it sounds to people outside of Japan.
So in case anyone was wondering, mystery solved.



