Enforcing that existential types use the any keyword will require a source change. To ease the migration, I propose to start allowing existential types to be spelled with any with the Swift 5.6 compiler, and require existential types to be spelled with any under the Swift 6 language mode. The old existential type syntax will continue to be supported under the Swift 5 language mode, and the transition to the new syntax is mechanical, so it can be performed automatically by a migrator.