When a database that has been operating with FCV=4.2 is downgraded to FCV=4.0, it should continue to operate. Ensure that:

• Existing unique indexes will be in V2Unique format, and updates to existing unique indexes will continue to be made in V2Unique format.
• Any new unique indexes will be created and used exclusively in the older format.

This may not require a code change but is worthwhile to test.