I would move it at the end of summer - late September/October when the ground is still warm and the roots can settle in before the worst winter weather. Lift as large a rootball as possible, Rhododendrons are usually quite shallow rooted. Add some compost to the hole and make sure it does not dry out. As it is thriving I assume you have an acid soil - if the leaves start yellowing water it with a sequestered iron compound. Good Luck!