i have moved a couple of over enthusiastic montanas before and usually when customers have insisted. I cut them both back to about 6' leaving some green on top. I dug out a huge amount of soil with the root ball enclosed within. Then moved it to the new location. The summers when I moved them weren't anywhere near as wet or cool as this one, yet they still survived. But I would always advise moving them when they are dormant. Between end of Nov thru to mid feb. You could ensure you have a plant by taking cuttings. so if you do move it and it doesnt survive you will have a back up that will quickly cover the new area.