More in Floors
Find and remove the existing nails.
Be sure to turn off the power to the room since there could be wires running under the floor or behind the wall. If you are unsure about the placement of the wires, have a spotter below to watch the blade as it moves through the wood.
Keep the blade at a 45-degree angle to the floor to make sure that you cut all of the plywood out from underneath the bottom
Snap a line along the center of the joist to keep it available as a nailer for the new subfloor.
Use a circular saw to cut along the line while riping out the rest of the nails.
With everything cleaned out, take measurements for the new subfloor. Transfer the measurements to the new 1/2" plywood, and make the cuts with a circular saw. Use a jigsaw if necessary to cut any indentations for the vent stack.
With the old floor gone, place a piece of scrap lumber across the joists to give yourself a nice safe place to stand on. The new plywood will be attached with construction adhesive and screws. Set the boards in place.