Four years ago, we had booked CX F HKG-BKK on a BA Avios redemption. The aircraft was changed to one without F. None of us had emerald status at the time but we did ask for and get F lounge access at HKG for that flight and also for a HKG-LHR J booking a few days later. We got the Avios difference back from BA, albeit 6 months later and BA threw in 20,000 Avios extra for their part in the delay in refund. So all in all, it was an excellent result.