If it's ever been painted your best to tear it down. If it hasn't been painted it should come off easily with water and a drywall knife. I used a garden sprayer to wet mine down and it came off very easily. I did have to touch up the seems and nail holes a bit but it finished very nicely. A lot...
I mounted mine the same as foss. I only have an 8 foot ceiling but because I chose to direct vent it straight out the wall I had to drop it low enough to get the right slope on the vent out the wall and below the garage roof eave. Something to think about if you are venting it out the side wall.