I've read Replacing a bathroom fan and Where can I find a bathroom fan that will fit in a 5" cylindrical space?. I decided that I would replace my Ventra bath fan with an Air King. The Ventra unit is 8" in diameter, and the Air King is 8.5" x 9" (without exhausts). The problem that I'm having is that I can't find how the Ventra unit is mounted:

I'm able to wiggle it on the right side, but the left won't budge. My attic space is not accessible over this area, should I just cut the dry wall around the unit? Any other suggestions or advice is appreciated.
Also, the Air King unit has a grounding wire; does this need to be connected?