"I would wait to transplant until after the shrub has gone dormant and dropped its leaves. This way there is less stress on it. If you have the time, the best thing to do is root prune next spring and transplant next fall after leaves drop. Root pruning means slicing into the ground around the plant, severing the spreading roots. Do this where you plan to excavate the rootball. Leave the plant in place, though, so that it can grow more roots within this imaginary rootball. Then, after a growing season, you can remove the plant and it should adjust more easily because it has more fibrous roots within that rootball.
Luke Miller, editor
Garden Ideas & Outdoor Living magazine"