Migrate from SQL Server 2014 Failover Cluster to 2019 Availability Group, rollback plan
2 Massive questions
You haven't said anything about what is connecting to your databases.
For swapping over the databases log shipping will give you a much shorter cutover time than backup &amp; restore.
If you can add a AOAG Listener with the name of the old server you will simplify updating all of the client connection strings (means the old server has to be powered down or renamed).
For rollback one generally has as thorough acceptance testing as possible - if that fails just power down the new server and power up the old one. Once clients are connecting and adding new data you have gone past the point of no return &amp; know you have to fix any problems that occur on the new server.