I would fill a media bag with some of the gravel, sense there should be alot of bacteria there from running the UGF.
I'd cover the top of the Bio-balls with this. And leave the HOB running for a while(couple of weeks?).
Depending on how much you clean your HOB. I've always "cleaned" mine when running an UGF, because I wasn't worried about bacteria up there.
You could always put a bag of gravel in their too.
With all of this, you should start right back up fairly quick.
I'd probably plan on leaving the bag(s) of gravel in for at least 4 weeks.
Or if you use multiple bags, you could start removing one a week at week 2 or 3, depending on how the tank reacts.
As in, if you remove one on week 2 and get an ammonia spike. I'd probably wait a couple of weeks before taking out another.