I would assume that even if OZ exits Star Alliance early, award tickets that are already booked will be honored. It may not always be the most accurate source, but I asked Copilot to summarize the conversion timeline for previous mergers (e.g. Continental/United, American/US Airways, United/Continental, LAN/TAM, etc.) and it seems there always was a grace period for award flights that are already booked of 3-6 months at a minimum.

My operating assumption is still that the part of Asiana/KE's announcement reading "Asiana Club members will continue to enjoy their current Star Alliance benefits until the merger with Korean Air is complete." means things will be as they were for all of 2026, but I am not 100% confident.
Personally, I soon need to pull the trigger on some vacation plans with AC for May/June 2026. Going even further out, I was hoping to take an LH or TAP flight within Europe in fall '26 to complete my final requalification before the merger completes. I am not loving the risk, but I am reasonably confident I can still get miles for the AC flight. For the other plan, I luckily still have plenty of time to wait and see.