The recent release of Revit 2021 has resulted in a change in the unit naming convention for some parameters. This has no impact for updated projects but has an impact on type catalogs where these parameters with renamed units exist.
If you try to load a previous Revit family version into a 2021 Revit project you will get an error message and the types will fail to load. If you upgrade that same family within a previous Revit version to 2021, Revit updates the unit naming within the background automatically.
My proposal is to include the same background function that exists for updating Revit projects for updating families as part of one of the upcoming service packs.