Losing a good seat! Is there a way to protect against seat changes?

I had 11A assigned on a 737-800 ATL-SJC for weeks. I still had it an hour or so before the flight. On boarding when my BP was scanned I got assigned a new seat - 10A.

I knew 10A was a hard bulkhead - not good for my 6' frame and bad knee for a five hour flight - leg needs to stretch out straight now and again.

I complained quietly to the agent who said the seat was reassigned to accommodate two passengers traveling together, one of whom was a wheelchair pax. I was offered non EC middle seats or exit row window as the only seats left if I didn't like 10A. I explained I too was a wheelchair pax and had wheelchair service between flights, so I doubt the system will let me take the exit row; besides it was non-reclining seat. Middle seats were unacceptable. So i suffered in 1)A and had a very sore knee after the flight.

There did not appear to be any compelling reason that this reassignment occurred.

So, any ideas on how one can protect against seat re-assignment outside of IROPS? Perhaps a note in PNR that pax needs this seat due to medical condition?

P.S. I did ask the isle pax in EC if they would change to a window seat and got no takers.