Whatever winter problems you have will be with the root exposure to cold in the boxes. (If they were in the ground, you'd need to do nothing at all.) You could try wrapping the boxes. Don't forget the bottoms if they are exposed to air rather than sitting on the ground. The grass will die back and in early spring you'll cut them back.

You have two choices since this is a plant which can survive down to Zone 5....

1. Replant the grasses in the ground....risky but could work if you do it now.

2. Leave them in their pots and hope for the best.

Generally when expecting a plant to survive in a pot, choose something that is at least hardy to 2 zones below your own....