API evolution is the process of migrating an inter-library interface from one version to another. Such a migration requires checking that all libraries which interact through the interface be updated. Libraries can be updated one by one if there is a transition period during which both updated and non-updated libraries can communicate through some transitional version of the interface. Static type checking can verify that all libraries have been updated, and thus that a transition period may end and the interface be moved forward safely. Anti-deprecation is a novel type-checking feature that allows static checking for more interface evolutions periods. Anti-deprecation, along with the more familiar deprecation, is formally studied as an extension to Featherweight Java. This formal study unearths weaknesses in two widely used deprecation checkers.