Remove the first node from the load balancer. All requests will be directed to the additional nodes. Check the $HOME/logs/request.log and ARTIFACTORY_URL/api/tasks (search for "running") to ensure that Artifactory is completely inactive.
Continue with the upgrade according to the instructions for your installation type.
Start up the first node. It will recognize that the HA cluster nodes are not all running the same version of Artifactory, and consequently, the system will be limited to allowing uploads and downloads.
Any attempt to perform other actions such as changing the DB schema, modifying permissions, changing repository configuration and more, are strictly blocked. This limitation will continue until all the cluster nodes are once again running the same version.
Version inconsistency generates exceptions
Running the HA cluster nodes with different versions generates exceptions. These can be seen in the log files and reflect the temporary inconsistent state during the upgrade process. This is normal and should be ignored until all the cluster nodes are, once again, running the same version.
You may need to update the Custom URL base as part of the Artifactory upgrade from 6.x to 7.x. Remember to use a URL that will work correctly in all cases; for example: https://mydomain, which is correct if the reverse proxy is also updated to use both the Artifactory REST API port and the Router port as documented.
Put the first node back to the load balancer.