Change the compatibility level of content databases for migration
When you migrate a content database from a SharePoint farm to another one, the compatibility level of the content database may be lower than the level of the destination SQL Server instance. This is because different SharePoint farms may use different versions of SQL Server. Because of this, when you restore or attach the content database to the destination SQL Server instance, the SQL Server version may be higher than the original SQL Server instance.
In this case, you can change the compatibility level of the database if the SQL Server version is supported in the SharePoint farm. However, you cannot change the compatibility level of the User Profile Application (UPA) databases because it is not supported.
Issue you may experience
If there is a large difference between the original SQL Server instance and the destination one, it may take a long time to execute the Mount-SPContentDatabase cmdlet to attach the database.
For example, if the compatibility level of the original content database is 90 (corresponds to SQL Server 2005) while the destination database is 130 (corresponds to SQL Server 2016), the Mount-SPContentDatabase process may take hours to complete. The process may be faster if you change the compatibility level of the original content database to 130.
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