Yes, has happened occasionally, broken seats and person with carer have been given reasons, although have been told I could use broken seat of I wanted to and in the carer case just ended up on the opposite bulkhead.

Edit: Ah, just re-read the op, I see what you're saying, you declined the upgrade and then found your seat assignment had been messed with. Could be connected, off chance of op-up at airport if insufficient people accept payed upgrade offer...