Good morning Chris,
Were the FSS Admin UI is still present in 12.5 it is not recommended to switch back and forth between using the FSSUI and the WAMUI. Once you start using the WAMUI you should use only that moving forward.
As was stated earlier if you have an R6 policy store that has not been extended and had the xpsobjects added to it than there is no way to use either the FSSUI or WAMUI from 12.5 to manage the objects in it without the risk of corrupting the policy store.
If you are in a migration phases and still have R6 policy stores and R6 policy server than you should use the Applet UI from the R6 policy server to manage the objects.
Please see the upgrade guide on when to start using the WAMUI in the migration process.
hope this helps
Gene