Action groups have become useful for things like extending solar panels, ladders, as well as toggling engines. I know how to set them up in the VAB, but I want to know if I can change action groups after I've launched.
Unfortunately, right now you can't. It has happened a lot to me, launching and then realizing I didn't remember to set up the keys to something. If you have to do something synchronized, you can try to pause/unpause the game while mousing from one component to the other to minimize the desynchronization.