I accidentally upgraded all my characters to another one.

I took all my players including the expensive ones and pressed this upgrade button, I didn't mean to.

By upgrade if you mean you have used them for training Ur stoichkov card.. that's bonkers but you can't undo it mate.  Keep your important players in a squad lineup so that they don't show up on training.. it's not that costly to buy a second lineup.

If u R saying it used up even the players on the lineup then it's a bug.