Cut the roots off. The plant will be fine, unless it's *Huge*. Then cut the top back a little.

Lots of mine are in gallon pots. I put them right on the ground & let the roots grow out the bottom. Then when it's time to move I cut the roots off, cut the top back & go to the new house.
Sometimes the old roots (still in the ground) will send up shoots.. Sometimes not. A little gift to the next renter.

I have a number of passies that I've sunk ingound in their pots. I just slice the roots and cut back the plant when I dig them up to bring them in for the winter. Before I plant them I close up the drainage holes on the bottom of the pot and cut new holes in the sides. That makes it much easier to lift the plants in the fall. You can just take a spade and slice off the roots that have grown out of the side holes.
Karyn