I’m still quite unclear on the basis for which a free SDC could be applied here?

a) standard redemption rules don’t apply (no changes within 24hrs) because a Joker has been played?

b) standard redemption rules don’t apply (no changes within 24hrs) because the usual redemption inventory of V or X is not in use

c) because the flight the OP was booked on was full

d) because it’s been done before

e) because BA should do simple things like this for high value, loyal pax

I’m not trying to be argumentative, I’m just unclear on why an SDC would have been a legal change in this circumstance