tsubasahome

IF the rose is growing okay, come dormant season you can actually dig it up, saw it apart at the base, and replant the different chunks. Much easier with own root plants! The trick with grafted would be to make sure each section gets a decent chunk of the main original root. You could do it over more than one season by digging down partway so you can see what's going on at the graft, and saw it partially apart but not completely, cover it in dirt again, and wait until the next year or two to dig and move the parts. In the meantime, you'd have to prune the more vigorous pieces to keep them restrained from drowning out the littler variety.