Since you mentioned plumbers, they are probably your biggest worry for structural integrity of the framing. In almost every old house bathroom I have re-done I have had to correct old framing weaknesses that were created by plumbers that cut the joists to heck to get their pipes in. If it looks like some of the old joists were hacked up then you should sister them to get the strength back. One method is to put 3/4" plywood on either side of the wound and glue and screw (or nail) from both sides. Also, be aware of any damage that your new plumbers might want to inflict on your structure. Plumbers are definitely not mechanical engineers and I have seen many floors that are being held up by some combination of inertia and plaster.