If you can, I would suggest just directly loading it in the latest version.
If the project uses things like third-party addons only available for the C2 runtime, then you may need to load in the last version supporting the C2 runtime to remove those things, save, and then reopen in the latest version.
Thank you Ashley, it worked. Since it's a huge project (the older version is on Steam, DinoSystem) there are still many little issues that need to be addressed, but will gradually do it.
PS: i found out that Play audio by name doesn't work if it is inside a sub-folder, will test again and file a report.