Toilets, Sinks, Showers, Tubs and Disposals - Replacing corner shower with bathtub/shower unit

We are interested in making an existing corner shower into a traditional bathtub/shower unit. Trying to figure out what all we need to consider in this situation. We may need some plumbing moved so we would have to have a plumber do that but we are hoping to do all the rest ourselves. The drain for the shower is in concrete slab for the corner shower, for a the bathtub drain would the existing drain actually have to be moved or would a pipe be able to run from the tub to the existing drain? We would tile from the top of the tub up most of the wall so would we remove the existing sheetrock and replace with cement backerboard? Would we put that all the way up to the ceiling or stop where the tile would end and use drywall above that, seems like a pain? The bathtub is going to be a tight fit in our small bathroom so I am wondering how much space we need between the bathtub and the studs, just room enough for the backerboard? Or do we need to plan space for plumbing? Any suggestions would be greatly appreciated.

When you mentioned "slab", all my thoughts changed. You may get lucky and the old trench will work, but highly unlikely. In which case you need to cut the floor as needed to run new pipes. And yes, I would definitely get a plumber for that. Getting into the walls is not that difficult. You will likely need to do some tearing out anyhow, for the new unit. And a standard tub is 60".