3.4 has a new feature that allows one to "Enforce Keyfile Access Control in a Replica Set without Downtime"

https://docs.mongodb.com/manual/tutorial/enforce-keyfile-access-control-in-existing-replica-set-without-downtime/

However, the documentation to "Enforce Keyfile Access Control in a Replica Set" still has this note:
"Enforcing access control on an existing replica set requires downtime."

https://docs.mongodb.com/manual/tutorial/enforce-keyfile-access-control-in-existing-replica-set/#downtime

That note is inconsistent with the new feature. Even if that link refers to a procedure that includes downtime, the downtime is no longer required.