Here is one method to replace the sub floor plywood under a toilet.
Move toilet with raised sub floor.
It is generally best to use a new wax ring when reinstalling the toilet.
Fit the toilet flange replace the subfloor and the finish flooring.
So your flange was already raised up to be close to flush with the tile.
I needed to move the drain about 2 inches and the hole i chipped out ended up over 1 foot across.
The concrete was only a few inches thick.
The first is that it connects the floor drain to the drain of your toilet.
I used an air chisel to chip out the concrete around the drain.
When the toilet is in place run a bead of silicone caulk along the seam between the base of the toilet and the floor.
This could be because of leaks or overflows.
Install the toilet closet flange on top of the flooring by solvent gluing it to the drain stub out and screwing the flange down to the subfloor.
Much of the work should be the similar for moving a toilet just on a bigger scale.
I think a greater danger to the sub floor would be from water leaking in from outside the toilet.
Cut off the drain pipe flush with the level of the finish floor using a reciprocating saw.
Toilet flange is a pipe fitting that basically has two functions.
This is a typical repair if your toilet has leaked long enough to rot the sub floor.
When using an extender set the toilet using a deep seal wax ring fitted to the horn of the toilet.
How do you move a toilet 3 inches with concrete sub floor.
The second is that it holds the toilet to the floor securely.
I moved my shower drain that is located in a concrete floor.
Consider moving the toilet to the shower corner in the current bath and adding a stacking washer and dryer where the toilet is now.
Ok you can fill the void with grout for a solid seal.
If you have the drain pipes and vents exposed you can build another floor for the toilet and have the toilet drain in the false floor traveling to the exposed stack vent pipe.
For example when the floor is mopped with a lot of water the toilet tank leaks or if the toilet.
Then convert the bedroom and laundry at the other end of the house into a master with ensuite bath closing that exit and finding a way to add an exit from the back of the house for access to the carport.