I've had to move smaller peonies in early summer and didn't kill them. Try to move as large of a root ball with surrounding dirt to give them a better chance and you might want to shade them a bit if they're going into a very sunny position, just until they get over the shock of transplanting. Even if they seem to die down, I've seen them come back the next year in the spring. If you're dividing and exposing the roots I'd wait until fall. Good luck!