Can a gravity flush toilet be swapped out for a pressure-assisted toilet? Are there any differences in plumbing, mounting, etc.?
It looks like there's no difference in plumbing requirements from this installation guide.
This regular toilet installation guide lists the drain and vent requirements:
it requires its own 2" minimum vent and a drain of at least 3" in diameter
As far as retrofitting a pressure assist vessel into an old gravity flush toilet, I wouldn't try it. Pressure assist toilets are designed differently than gravity flush ones. Flushmate's website warns:
The FLUSHMATE® Pressure-Assist system cannot be retrofitted into a standard, gravity-fed toilet.
There might be a retrofit kit out there somewhere, but I'd be wary about how well it would work.