I thought I had my answer per another post but just to be the same answers apply to an upstairs bathroom....
We had to tear out the plywood flooring in our upstairs bathroom to get rid of mold (treated and all that, mold = gone!). He took out two full sheets of plywood and replaced with 3/4 inch treated sheets. The existing plywood, still healthy, measured 3/4 inch, there's only a small strip on the far end of the bathroom left.
These were attached to braced beams below.
Naturally, the new plywood is probably 1/8 to 1/4 inch thicker. We plan to put down cement board and ceramic tile.
In a prior post, it was suggested to level plywood difference with a cementious based floor patch / leveler. Is this still the product and method to use in our case?
Thank you. If anyone has questions about creating a publication for print, I can help with that.