You're right, they usually install the bathtub and then build the finished walls around it. You'll probably have to remove some of the drywall, since it probably covers a flange at the top. When I replaced mine, I just took a sawsall and chopped it up into manageable pieces. Be careful around the supply pipes so you don't accidentally nick one and create a flood. Note that the size of the thing is probably fairly standard (60" long, stud-to-stud) but the drain may not be in the same location...it'll be close, but you may need to move things.
Each of the tubs has their own installation requirements...best to find the one you want then read up on the installation instructions. You may need to put up drywall to hold the surround, or it might just go directly onto the studs.