There was a soft spot by the door that bothered me every time I walked over it.  It was a sound piece of floor, just gave a little. I had not replaced it during the initial floor repair stage because it did seem fine.  Since the linoleum will be installed soon, I just had to do it now or regret it forever.

With the benefit of past experience, it took only an hour.

• I roughly located the supports underneath, then cut a 8" by 8" square in the center of the section to be replaced.
• I put my arm in and felt around for the exact location of the edge of the 1x4 attached to the beam, put a tape measure in and figured where the center of the 1 x 4's were, marked the lines on the floor and cut with a skil saw set just deep enough to cut through the plywood and leave the underlying 1 x 4 intact. it is important have the edge of your new board come the center of the 1 x 4 board for best support on all sides.
• Once I pried the panel up with a crow bar, I had to remove some of the big screws (I know the panel would come up more easily if you take the screws out first, but they are gooked in there pretty well).
• To remove them first try with drill with #3 phillips head (yep, it must be a #3) , then try vice grips, if that doesn't work, just grind it off with a grinder.
• The 1x4's were like new so no replacement needed, but the screws removed have to be replaced
• The original screws went through the plywood and the 1x4 and tapped right in to the steel frame.  It will work just as well to use self tapping metal screws to to go just through the 1x4's and attached the plywood to the 1x4's with 1 1/2" wood screws.  Longer ones will bottom out on the frame.
• Cut 3/4" plywood to fit
• Screw it in, then caulk, cover or whatever.

With new panel in place