The reason you need to do this is because otherwise the manager has no knowledge of what happens with users that log off while the Appvolumes manager is not available.
We have 2 Appvolumes managers behind an F5 and thus we upgraded them one by one. No need to remove all active attachments.
When using the upgrade to 2.12.1 it doesn't ask you to remove the appstacks. We did a rebuild of the database (had some obsolete records due to upgrade from 2.6 to 2.9) so we installed it freshly. My guess is that you don't need to detach all appstacks when upgrading from 2.12 to 2.12.1 because the manager is always active.