This just happened to me.

Booked QR Y using BA miles while I was BA Silver. Selected front row seats for free with zero mention of charges or fees.

Now BA Bronze and was asked to pay at the check in counter. I was happy to take different seats but the flight was full. Escalated to a manager who said we could not fly without paying despite already having electronic boarding passes (we only stopped by the counter to drop off bags). Manager ended up relenting after some back-and-forth but required overriding from the back office. Not sure what would have happened had we not needed to check bags. Would they have denied us boarding?