Can a commercial cargo aircraft be converted into a commercial passenger aircraft, and vice versa?
There are such a thing as "Quick Change" aircraft, allowing entire 'pallets' of chairs to be exchanged for cargo.

Image courtesy of canalblog.com

Image courtesy of airliner.net

Nice film to show the process.

Military transport aircraft can also be fitted out with stretchers or seats, or whatever other combo of stuff you want. To the best of my understanding these lock in like any other cargo pallet in the floor. They are less concerned with aesthetics and more with practicality:

Image courtesy the DOD via Wikimedia

Image courtesy of aarcorp.com