Prefacing this by saying I am not the expert and I haven't necessarily had to do this, but I am experimenting this year with late season divisions, on which there were few roots and no soil. Your situation is quite a bit different and most likely more forgiving.
I think, in general, as long as you don't cut into the corm at this late stage, you are probably fine cutting back the surrounding root mass, within reason, to a manageable size. If you supplement the new pot with additional mix to fill any gaps, I would make doubly sure the mix is very light, airy and drains extremely well because its very possible there may be little, if any, root growth into the new medium which could leave it wet for prolonged periods. We all know how much dormant bananas like wet.
If it were me and it had to be done, I would consider doing it now, then keeping it in a bright, warm location, supplemental light and heat if needed, for 30 days or so until it stabilizes, then move it into its dormant location.
Of course another possibility is to dry store it. That would involve digging the plant out, taking most of the potting medium off of the corm, cutting the leaves off and storing it in a cool, dark place for the winter until it's time to put it back into the pot, or ground for that matter.
Here's a good thread to read through if you haven't:
Time to put the bananas to sleep for the Winter
But hopefully one of the expert northern container growers will chime in with their suggestions.
Good luck and keep us posted!