I'm not a rose specialist, but in my own experience, a plant will usually drop its flowers or buds if the roots are disturbed. The roots need to re-establish themselves in order to support the flowers. You could probably move it immediately after it flowers. Better yet, wait until fall and move it as it is going dormant for the winter.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams