Answer: Volunteer plants typically have deeper root systems than cultivated and transplanted plants so that's a consideration when you're planning on digging and moving. Butterfly bushes are pretty resilient, though, so I think you can move it without too much difficulty if you want until winter when it has slowed its growth. I would prune it back to a manageable size in January or February, dig it up, wrap the roots to keep them from drying out during the transit, then replant and water it in well. It should take a few months to establish and then should grow well in the spring.
Best wishes with the move!
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