Like Jay said, cut what you can with a power saw set to the depth of the floor. Either hand chisel, carefully angle hold a reciprocating saw, or use a dremel/rotozip tool with a cutting head to cut the edges and go under the wall a ways if you want. Using a reciprocating saw or rotozip tool can be very wicked and kick around on you so it takes a firm and experienced hand. Have extra saw blades (the short ones) for the ones you bend or break.
I'm not too concerned about nailers along the edges just like Jay said and you have lots of edges on your floor without nailers. Think about construction glue on your subfloor joists and using screws to help avoid squeeks later. You may have to scrape your joists to get any old glue and wood residue off of them. Keep in mind that new plywood may be a 1/16" or so thinner than it used to be. Use roofing felt for a spacer if needed. Good luck.