Rose Tree
Doug says that replanting it can be done when the tree is dormant (no leaves or swelling buds in the spring). Get as many of the roots as you can (with a big plant like this, it's going to be a massive root ball)
If you're not a gardener or rose person, there's no way I'm going to be able to tell you how to graft a new tree rose. You need proper rootstocks, trunks etc and budding material and the skill to do it. I'd suggest you contact the local horticultural society (you don't say where you are) and get some advice from local folks as to who in the area might be able to help you.
As an aside, tree roses require pruning like any other rose and dieback is can be expected as plants grow and require care. I'd also suggest you read the pruning directions on the rose section and pick up some local courses on it to get your rose as healthy as possible without having to transplant or graft it.