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The following page summarizes various considerations for downgrading to 4.2. For specific instructions for your deployment type, see:
Before you upgrade or downgrade a replica set, ensure all replica set members are running. If you do not, the upgrade or downgrade will not complete until all members are started.
Once upgraded to 4.4, if you need to downgrade, we recommend downgrading to the latest patch release of 4.2.
If you need to downgrade from version 4.4, downgrade to 4.2.6 or a later version. You cannot downgrade to 4.2.5 or an earlier version.
Optional but Recommended. Create a backup of your database.
Before downgrading the binaries, you must downgrade the featureCompatibilityVersion (fCV) to "4.2".
Before downgrading the binaries, remove all persisted 4.4 features that are incompatible with 4.2.