Posted: 01/08/2014 at 15:35
Jonathan..........in my experience, most Hydrangeas are best grown in the ground. If yours is suffering from being pot bound there doesn't seem to be any advantage in taking it out and replacing it in the same size pot.
If there is really no alternative, you could prune the top growth down, reduce the size of the rootball and put back in the same pot with new compost and keep well watered. I wouldn't like to say that you would end up with a good specimen tho.
Someone else may have a better idea or method