THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
...
If users choose not to support plain-text (for security reasons), old brokers and clients will be unable to connect - but this is expected.
(To clarify the last line, because I just got bitten by this:
If your listeners do not contain PLAINTEXT for whatever reason, you need a cluster with 100% new brokers, you need to set replication.security.protocol to something non-default and you need to set use.new.wire.protocol=true for all brokers.