This plugin will help you to check whether you can upgrade directly to the next major version of Elasticsearch, or whether you need to make changes to your data and cluster before doing so.
The plugin didn't raise any issues with my k4 index but while upgrading I hit:
Oct 30 05:28:53 jotunheim elasticsearch[14110]: Exception in thread "main" java.lang.IllegalStateException: unable to upgrade the mappings for the index [.kibana], reason: [Mapper for [_index] conflicts with
Oct 30 05:28:53 jotunheim elasticsearch[14110]: [mapper [_index] cannot be changed from type [_index] to [string]]]
Oct 30 05:28:53 jotunheim elasticsearch[14110]: Likely root cause: java.lang.IllegalArgumentException: Mapper for [_index] conflicts with existing mapping in other types:
Oct 30 05:28:53 jotunheim elasticsearch[14110]: [mapper [_index] cannot be changed from type [_index] to [string]]
The plugin didn't raise any issues with my k4 index but while upgrading I hit: