You do not want to move or transplant plants during the heat of summer and the peak of the growing season if you can possibly avoid it. Too much stress on the plants. Wait until fall and cooler weather or when the plant is dormant.

And careful about dividing. While most hydrangeas are caning shrubs, shrubs do not divide in the same manner or as easily as perennials do. When you do dig them up, you might be able to gently separate away some of the most exterior stems that emerge from the ground. But don't whack through the rootball or you risk killing the plant.