Flooring Tile - Retiling ceramic tile floor - Would I have to remove entire substrate or just tiles?

I have a ceramic tile floor in the bathroom that I would like to retilel The tiles are set on a concrete slab substrate. What I am wondering is, if I remove the tiles, will I be able to reuse the mortar bed or will I have to remove that and start fresh w/ plywood and backerboard? Would I be able to remove the thin set and get a nice flat surface for the new tiles otherwise?

If none of the old tiles are loose or cracked and the added height is not an issue you could go right over the old tile by super cleaning them of any soap, spray, dirt, grease, etc then using a modified thinset to set the new tile.
If they must come out rent a chipping hammer with a 2" spade. All of the tile and most of the thinset will come up quickly. As long as the old slab does not have cracks running through it , it is reusable, even with gouges from the demo which can be filled in with thinset prior to setting the tile.

Thanks. I know that going over the old is an option and I've been battling the decision for a few weeks. Thing is, I'm one of those tear-it-out-and-do-it-right type of guys.... I could just extend the toilet flange and threshold the door, but I'm thinking maybe I should just try to remove the old tiles...

Nothing wrong with that plan as long as all conditions are met, in fact, it is an approved method in the TCNA Handbook for the Installation of Ceramic Tile. Its a sturdy flat surface to work off of. But then again the out with the old does have appeal.