Switch Hitters

if not mistaken, only if the opposition changes pitchers in the middle of an at bat...

I believe that you’re mistaken. I googled it (not able to link, sorry) MLB batter can switch at any point during the AB EXCEPT during the pitchers’s windup.
There’s a slight problem with changing over the helmet equipment for the batter as the earflap would have to cover the other side of his head from where he started.

i've only seen batters clown around like that in all-star games, so i couldn't remember if it was legal...

If you google that possibility it states that sort of switching is legal. That isn’t to say that it’s a wise practice. Most switch-hitters are better hitters from their dominant side.