If you have multiple servers associated with a driver set, you can perform an upgrade or a migration on one server at a time. If you don’t have time to upgrade the servers at the same time, the drivers continue to work with the different versions of Identity Manager until the upgrades for each server can be completed.
The Identity Manager engine is backward compatible, so the Identity Manager 4.0.1 engine can run Identity Manager 3.6.x drivers without problems.
IMPORTANT:If you enable features for drivers that are supported only on Identity Manager 4.0 or later, the drivers stop working on the servers with mixed versions. The older engines cannot handle the new functionality. This breaks the drivers until all servers are upgraded to Identity Manager 4.0 or later.