This happened to me in December last year from F to J, HKG-MXP. They changed planes a week or so before the flight and pushed me down to J without letting me know first.

Countless calls to different managers in Manila before getting through to someone in Hong Kong office who was a manager. They reimbursed me partial miles (16k or something like that), but not what the difference would be if I had booked it in J to begin with. The manager was able to pull a few strings on the ground in HKG though, and got me access for me and my travelling companion (who was in Y) to the F lounges, but it took A LOT of negotiating to get that.

I feel CX's standard protocol is usually to offer nothing and see what they can get away with. Not great.