Preparing bathroom wall for tiling

• Step 1: Remove existing fixtures (toilet, sink, etc.)
• Step 2: Remove existing tile.
• Step 3: Determine if existing sub floor is appropriate for tile.
• Step 3 a: Replace existing sub floor (if necessary).
• Step 3 b: Install appropriate sub floor.
• Step 4: Install new tile.
• Step 5: Reinstall fixtures.
• Step 6: Enjoy your new bathroom for years to come.

This may not be the answer you are looking for, but to do the job right (in my opinion) you should start fresh.

If you don't feel comfortable doing the job yourself, you might think about hiring a professional.

I apologizes for the bluntness of my answer; as a home owner myself who has had to fix the messes others have created, in my opinion there is no excuse for not doing a job properly and (as Mike Holmes would say) making it right.

Removing the existing tile will cost you nothing but time, and in the end it will make you feel much better knowing that you can enjoy the remodel for years to come trouble free.

You've got a sticky wicket of a situation. However, being able to remove the pipe flange was a great benefit. You'll need to build-out the joist under the wall with layers of 2x6 scraps until the scraps protrude at least 3/4" beyond the wall plane for good support of the new subfloor & the ability to screw or nail.

To cut out the old subfloor, lay a bridge of plywood over the wood area, from Gypcrete to Gypcrete. Drill holes in the wood area, inside of the joist, to poke a tape measure down. Then, set your circular saw's blade depth to the measurement from the bridge's top & down to the bottom of the subfloor.

Then, you'll want to remove the nails & cut 3/4" over or on top of the joists (likely a line where the old nails were), so the new subfloor can rest on half of the old joist. Once the wood floor's out, you can use it as a template for a drop-in plywood replacement. You can do cement or cement board on top of the new subfloor patch.