I bet it doesn't look this bad?
That 'dark' spot is where our shower 'was'.
I understand exactly what your asking.
I ripped out al 10'x14' section of sub floor.
I did my best to cut the existing sub floor 1x6's on the middle of the floor joist. Once I had the new floor joists in, I nailed down new 3/4" struct plywood. I did a good job of edge nailing all of the existing 1x6's.....in retrospect, I think I would have been better off running a second 2x8 sistered up to the joist the existing sub floor was on...and cutting the existing 1x6's flush with the outside of the 2x8. That way the diagnal 1x6's would have the full width of the floor joist...and the new plywood would sit on it's own joist.
For reference....I'm building a 2-story addition now....part of the 1st floor is raised foundation with a subfloor.....I'm using 3/4" T&G glued and nailed. It is also on 2x8 floor joists......but I gotta tell ya.....that gluing sure seems to make the floor a bit more solid as compared to the existing house.