@Dave0x1 
Apache NiFi takes carful consideration when upgrading/migrating from an older version to a newer version with in reason.  Like upgrading from 1.13 to 1.19 would be expected to pose no real issues; however, jumping from 1.10 directly to 1.19 may pose some challenges.

That being said, rolling back is not something that can be guaranteed to go smoothly as some versions can result in changes to database structure/content, modification/change to NiFi components (processor, controller services, reporting tasks, etc).  In a best case scenario, a rollback my result in a working NiFi, with unexpected unexpected invalid components on the NiFi canvas.  This happens when new properties are introduced in  components being used, Those component changes get written to the flow.xml.gz and new flow.josn.gz files.  So on rollback those new properties turn in to dynamic properties which may result in an invalid component until they are removed manually.  In other scenarios, there may have been upgrades/changes to H2 DB that won't rollback smoothly.    So perhaps taking a backup of the flow.xml.gz. flow.json.gz, and database_repository/ directory contents would be useful in the vent of rollback. 

Good news is that there is nothing in the H2 DBs located in the database_repository you need to retain.
The nifi-flow-audit DB tracks history of changes you made on the canvas.  The other user and identity dbs can just be removed and allowed to be recreated on next startup.

So in your rollback test, I would rename the existing DB files and restart allowing new to be created.

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Thank you,

Matt