When Cluster Has Nodes at Different Software Version Levels
The rolling upgrade feature is designed is to enable business continuity while a cluster is being upgraded. The window of time when a cluster has nodes of varying versions should be small. During this time, do not make application code changes and/or configuration changes.
Configuration changes involve the following:
Changes to index, forest, database, application server, host, group, and cluster setting
Changes to security settings such adding/changing/deleting roles, users, privileges, credentials, certificates, etc.
Adding/removing/updating TDE templates
Adding/removing/updating redaction rules
In addition, do not perform any manual merges and disable reindexing while the cluster has nodes that are at different software version levels. Changing error log settings and adding trace events to debug issues should be fine.