Hi @Cristian Mitan ,
> we have a 2 node SQL Server 2014 failover cluster with 2 SQL Server instances and about 100 databases to be migrated to an availability Group 2019.
What did you real mean? SQL server 2014 failover cluster instance is a instance level HA. SQL server 2019 availability group is a database level HA. They are different. Please refer to the blog Always on Availability Groups vs Always on Failover Cluster Instances to get more information.
Or if you have SQL server 2014 AG and want to upgrade AG replica instance from SQL 2014 to SQL2019? Suggest you rolling upgrade for AG replica instance. Refer to Rolling Upgrade Process to get detail steps.
Or if your AG contains failover cluster instance (FCI) nodes, you should upgrade the inactive nodes before you upgrade the active nodes. Refer to AG with Failover Cluster Instance Nodes to get detail upgradation steps.
If I misunderstood your question, please let me know. And please describe your question more clearly to make us better understood this question .
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