This could be fairly easy, or a big pain in the ***. A shower drain line is usually 2" by code; a tub only needs a 1.5" (and that's what all of the drain kits are designed to mate to). That in itself isn't a problem, as there are adapters to go from one size to the next. The biggest problem is moving the supplies and probably more so, the drain and vent. You'll probably need to tear up the floor or the ceiling. Once you have one or the other opened up, you'll be able to see where the pipes run, and the best way to move them. It's really hard (next to impossible) to tell you how big of a deal this is without seeing the current actual pipe runs and the joist/wall structure layout. The drain needs to slope down 1/4" per foot from where you set the trap to where it then runs to the waste stack. Achieving that depth can be a pain, and the direction may be a problem if it crosses joists. One alternative is to run the pipe below the ceiling and then box it in in a chase, if you can't cut through joists). It really all depends on how it currently is layed out.