I started in the upstairs bathroom last weekend. I took the trusty sawsall and just cut out the entire shower. What I found was as bad as I had expected. I have 2x10 floor joists and 3/4" plywood for the floor. On top of that was 1/4" lewon plywood and ceramic tile. The leak(s) from the shower had gotten between the lewon and the 3/4" plywood. It was quite rotten so I just pulled up all of the tile and lewon.
As a side story, we have had problems with palimino bugs in the kitchen. The kitchen is underneath the bathroom. Well, I could never find where the bastards were coming from. These pests need moisture to survive. The kitchen and underneath the house are dry. When I pulled down the shower I exposed a nest of the bugs! That is where they were getting their moisture. So, at least that mystery is solved.
Also, the water pipes that go to the shower come in from the ceiling of the bathroom, down the shower wall and, for some reason, into the floor under the shower. Where the hell were they going? They went to the bathtub on the other side of the room. I cannot fathom why it was plumbed this way because the tub and the sink are on the same wall. But instead of coming off of the sink to the tub, the plumbing was routed under the floor from the shower.
Anyway, the 3/4" plywood where the shower was will have to be replaced but only one piece. So that's pretty good. However we have not decided whether to just replace the shower or do we do a complete change up of the layout of the room. The bathtub is a deep soaker tub with jets and it only gets used two or three times a year. I don't see any reason to keep it. A shower is all we need.