Ducting Systems and Air Ventilation - how to remove duct fitting without destroying it

I'm in the middle of a renovation project and I need to "flip" one of the register fittings on a 6 inch duct.

Currently, it points to a room in the upstairs but I want to turn it over so it points to the basement instead. The reason is that it used to sit at an outside wall under a window but now is in the middle of a much larger, newly constructed, room. I'm extending other ducts for the new room upstairs but would like to use it to blow heat/cold into the room below.

Is there a way to save the fitting? It seems to be riveted into the floor sheathing so I imagine I'll have to cut it out but I'm hoping there's some type of adapter that will extend the part that I removed. I haven't been able to find anything at Lowes or Home Depot.

Thanks

You can get a new boot for a few bucks. You will most likely trash the old one trying to get it out.