Can I switch which muscle I attack?

Yes, but it starts that fight over. Switching to a different muscle clears the stars you already earned on the old one and gives you a fresh target.

Kovo warns you when you have stars to lose, so you can back out before it happens.

The new muscle comes with its own XP number. It can be higher or lower than the one you left.

Going back to the muscle you left needs a new workout. The stars you had there do not come back on their own.

Learn more: How war stars work