Answer: I'd wait until frost kills the tops back and the shrubs go into dormancy before digging and transplanting. Or, you could do it in early spring, before too much new growth appears. How extensive a root system really depends upon your soil type and how they've been cared for. I suspect the majority of the roots of a shrub that's been in the ground for two years will be directly beneath the shrub, extending 12-18" into the earth and out slightly further than the longest branches. The rootmass may be smaller, but start digging there and work your way in towards the main stem.
Be sure to plant at the same soil level as the shrubs were growing before and water well to help settle the soil after you've transplanted your butterfly bushes.
Enjoy your garden!
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