fix the old legacy replay they desync
fix the old legacy replay
What even are "legacy" replays? November 2020 replays that you open through the legacy (python) client? Replays from more than 1 year ago? Replays before 2014?
If you want to watch replays from before the current balance patch, you may need to disable all UI mods. UI mods that are designed for the current version of the game are probably not going to work on older replays.
I have no idea what is necessary for replays prior to 2014. Maybe you would need to manually set up the game files for those very old balance patches in order to run the replays yourself. For example, the devs apparently have a way to run a version of the FAF client that is offline or at least that doesn't check in as normal with the main FAF server. I think they primarily use it to test "beta" stuff before they commit an update to the beta branch so they don't break everything. Perhaps you could repurpose the offline version of the client to load ancient versions of the game in order to watch replays from pre-2014.
If you want to watch replays using the legacy client (the Python client) . . . I guess you need to learn Python, or you need to get some money together to hire a Python dev. It's fine to ask about it on the forums, so you can see that nobody is going to step up and do it for you.