Just by putting the plywood subfloor over the hardwood, the hardwood will be damaged with a bunch of nail holes. You're going to have to address that issue if you decide to rip everything up. Hard to tell how that will look and hopefully, they didn't use any mastic or glue between the plywood and hardwood.

What you might consider is just taking up the plywood and then leaving the hardwood alone and using it as a subfloor. Go out and buy some new flooring that is close in appearance to the rest of the house. Lay that on top of the hardwood. You will probably have a nicer job that way.

Good Luck