In our Documentation for rotating keyfiles in a replica set, we provide two methods for specifying the old and new keys in the keyfile, one using "Multiple Key Strings" and the other using "Multiple Key Sequences".

A customer opened a case with us because they were using the "Multiple Key Strings" approach but when they reached step 4, the first secondary they tried to restart when the keyfile only contained the new key, failed to rejoin the replica set.

I've also tested this and it works when using the "Multiple Key Sequences" approach, but I get the same result as the customer when using the "Multiple Key Strings" approach.

Customer is running version 4.4.6.

Please let me know if you need any more information.