This is a very timely thread.
I'm in the exact same situation as the OP, having had my TG F class flight SYD-BKK downgraded to C class.
I initially rebooked SYD-BKK-ICN-LAX to SYD-PEK-ICN-LAX on CA (3 hour layover in PEK, 3 hours in ICN), but I have now changed back to going via BKK now that F class is once again available. Not that I think that CA F class would be disappointing, but as pointed out earlier in the thread, connecting at PEK in the winter may not be the best idea...
Having 2 hours in the F class lounge in BKK and 8 hours to explore Seoul is better than risk getting stuck at PEK