What I have found out about butterfly bushes is that they are extremely hard to kill. My father in law had one like you are describing. He not only transplanted it but split into two
plants about a month ago. He cut it back to about 3 or 4 ft. tall, and then kept it well watered. It did go into shock, but both
plants are now blooming again.
Some experts are even trying to get it listed as an invasive
plant, because it is so hard to kill.
That said, if you can wait till it goes dormant, then dig it up and transplant it. If you can not wait, then I would trim it back and then transplant it, and keep it well watered. It may not produce any more
flowers this year but it should be fine by next year.
I hope this helps some.