The best time to transplant hostas would be in very early spring before the hostas have come out of dormanct or in early fall after frost has sent them into dormancy. Anytime you transplant them after they’ve leafed ot, they are going to do the “flop”. I wouldn’t cut them back since this will shock them even more than transplanting them at the wrong time. The best you can do to alleviate some of the shock will be to keep them moist as they try to survive the hot days of summer ahead,