The page at https://docs.mongodb.com/v3.2/release-notes/3.2-upgrade/#upgrade-a-replica-set-to-3-2 does not suggest enabling replication protocol v1 after upgrade is considered final (no downgrade is required). Without it enabled, RS improvements done in 3.2 (https://docs.mongodb.com/v3.2/release-notes/3.2/#replication-election-enhancements) are left out in upgraded sets. I believe the documentation should state that after upgraded replica set is considered stable, the v1 protocol should be enabled, possibly listing an example of how it should be done.