Do you use Ambari? If so, then there should be no problems as the service user information is recorded in Ambari and propagates through with upgrades. If you don't, then you probably just have to make sure you copy across any relevant configuration parameters, remember to start your services as that user and check HDFS permissions are correct as part of the post-upgrade validation process. As you say, custom users are totally supported, so you shouldn't experience any problems. ... View more