Tile – Tiling on greenboards

If your tile is not secure, then the backer is not secure either. You will need to gut it to the studs and install water proof membrane, concrete board or heavy hardy-backer to tile over. The window frame must also be carefully water-proofed as well and trimmed out with a moisture resistant material. Get some professional advise on this one, a new tile job is not cheap or easy and be a shame to have a leaky, grout dropping job when you're done.

As far as the type of tile, ceramic or porcelain are similar products, difference being the hardness grade and the type of glazing. Porcelain is always a better choice in my view as the surface glaze is smoother, easier to keep clean and usually a harder grade.(3 to 7) prices can vary wildly, usually again because of the glazing and hardness factors. Designer patterns also effect the cost as well.

• 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.