alternatively if that was too much for you.. you can go the focus route..

make all your macros use the

/target [target=focus,exists]
/assist
/cast some spell

and bind

F10 to a macro:
/target MrA
/focus

F11:
/target MrB
/focus

F12:
/target MrC
/focus

etc

but:
for MrA make F10 a /clearfocus macro
MrB make F11 a /clearfocus macro
etc...

that way when you switch main.. he wont confuse himself by trying to focus himself/herslef and assist.. :S assisting myself would be too confusing :S

then whenever you want to switch you just press the button and they all switch assists.