Changing the subtitles would be trivial in terms of effort. It would even be unnecessary if the change was just a couple frames (e.g. to the 4Kxx timing). If Harmy decides to change to a standard that’s neither GOUT nor 4Kxx (which would fix at least two issues with the current situation*), then subtitles could still be used, with an offset (and so could some audio). Rendering the subtitle graphics could still take a week or so, but that’s a fire-and-forget process. All I need is lead time.
Audio would take some work to modify, no doubt about that. Much advance notice would be nice.
* the HMDI handshake muting part of the fanfare on some systems, and users not knowing if their audio is properly synced or not, even while watching.