I'm no expert, but here's my experience with moving buddleias. I've moved my one dwarf from the ground, to a container, to the ground, and to it's final location...in the ground. I've also moved a large Royal Red from the back of my house to front. I think that each move occurred in early spring. Some before the buds broke, and some not too long after. I would cut the branches all back to around 12 inches and dug up as much of a rootball as I could.

With the large Royal Red I was much more brutal. It had roots over an inch and a half thick in spots. I just broke through them with a shovel. There was no rootball. Just a bunch of roots. The soil didn't hold around them, but I planted it quickly and watered it in well. I do admit that the bush looked alright that season, but not nearly as leafy or flowerful as previous seasons. That was last season. Right now it is extremely lush like other years. It seems to have sprung back well.

Butterfly bushes appear to be VERY resilient. Even so, I would wait until early Spring if you can since you probably already have a large plant nearing blooming size. That would put a lot of stress on it. Best bet: Move it just after you prune the branches down in early Spring. Around here in zone 6b that time is March.
Kim

Here is a link that might be useful: My veggie and flower garden

Another idea is to take cuttings from your butterfly bush and root them. These plants grow so fast and the cuttings will root quickly and bloom the first year, at least, the one's I've had experience with do. If the cuttings don't grow well for you, then you can still attempt to move the original plant. Best of luck with your project.

Mary

Susan, My experience has been that they transplant very well and seem to bounce back really quickly with just a little extra attention. I am always amazed by how resilient they are. I always try to get as much of the rootball as I can anytime I tranplant anything.~~Angie

Thanks for the great everyone! I think I will take a page from each book - a cutting or two just in case, and transplanting. Sandy advised that the roots on a Buddleia grow out (fibrous) rather than downward. Also, I should have the planting hole ready for the plant so as not to leave it sitting somewhere while I dig one.

Woo hoo! I rarely transplant anything so I am truly a novice when it comes to that.

Susan

Well, this one is almost a sure success Susan. Just don't forget to give it a little water now and then until it reestablishes. I always dig my new hole first, and I also make sure to really break up the dirt in the bottom of the new hole so that the roots will have an easier time getting into the soil there.

maybe i am getting old and slow, but when i can get one for under 7 dollars at bluestone perennials, I would leave the one in the shade and buy a new one and plant it where you want. but then again, my age is showing this morning.
plus, i have moved plants around for the past 30 years gardening and my back hurts today. LOL.

You can probably get a much larger one than you can get online at Lowes for around the same price as well. :)

The Buddleia Susan has is the bicolor. It's very hard to find. So, she's wanting to save it.

Sandy