Can't replace hardwood floors, what are my options?
Yes you can use the existing floor as a subfloor, but I humbly submit, that the brittle cement based self leveling underlayment would not do well sandwiched between two wood surfaces, and a slew of fasteners driven through it. I seems to me the vibration, the fastener shooting through the cement would start cracking up the poured underlayment. I believe it is done a lot as a standard operating procedure, so it is accepted. I would not want a bunch of cement chips holding up my floor. But once it is done, it is always out of sight, out of mind, I guess has been the norm too. I am not trying to slam anybody out there, I see that the accepted "norms" are changing. If I am wrong please say so.

If your floor is under the interior walls and most of the damage is in the middle of the rooms where all the foot traffic has been, and you know your existing floor is 3/4", I would cut out the sections of damaged floor and leave the existing intact flooring, as much as possible, to pass under the walls. Wall supported by joists (perpendicular) are not as critical, you can remove the flooring as close to the wall as you like. It's the flooring where the walls are supported in the direction with the joists. Keep the existing flooring intact for at least two joist spans and cut out the rest, replacing it with 3/4" CDX for 16" centers or 3/4" T&amp;G for 24" centers. You could use T&amp;G for all of too if you like. If your existing flooring is 7/8" or 1" thick, add an appropriate shim on the joists before you lay the new subfloor. Where the plywood meets the second joist back from the parallel wall, at minimum, add a 2X4 nailer,  slightly elevated to make the new and old surfaces flush. Glue the nailer in place, using screws or nails to hold it until the glue sets.

Using an adaptation of this process, you can even flatten the floors a bit while the damaged floor is up, you could sister the existing joists with 2X4's to eliminate the sag that the existing joists have created over their years of service. You will not need to add the shims then, just raise the sistered joists high enough to make up the difference in the thicknesses as well as creating a new flat surface. A scrap of 3/4" plywood will set the ends right, and a string line through the middle, raised slightly above the nailers at the ends will get the center of the room flat. There is a little more to it, but I am trying to keep from writing a novel. This should get you going in the right direction.