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There're some caveat of upgrade and downgrade protocol version.
1. They should (at least downgrade) should happen in isolation with no other config changes. See SERVER-21744.
2. With old drivers released before DRIVERS-279 is resolved, clients have to restart to observe the primary after downgrade.
3. Re-upgrade after downgrade shouldn't start until all nodes are in protocol version 0 by using rs.status(). Otherwise, the re-upgrade may observe oplogs in previous upgrade, rolling back new committed oplogs.
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