SQL Server 2016 -> 2019 AlwaysOn Rolling Upgrade Migration
This is from Microsoft books online :

Initialize secondary availability groups in a distributed availability group
Distributed availability groups were designed with automatic seeding to be
the main method used to initialize the primary replica on the second
availability group. A full database restore on the primary replica of the
second availability group is possible if you do the following:


Restore the database backup WITH NORECOVERY.
If necessary, restore the proper transaction log backups WITH NORECOVERY.
Create the second availability group without specifying a database name and
with SEEDING_MODE set to AUTOMATIC.
Create the distributed availability group by using automatic seeding.
When you add the second availability group's primary replica to the
distributed availability group, the replica is checked against the first
availability group's primary databases, and seeding catches the database up
to the source. There are a few caveats:


The output shown in sys.dm_hadr_automatic_seeding on the primary replica of
the second availability group will display a current_state of FAILED with the
reason &quot;Seeding Check Message Timeout.&quot;


The current SQL Server log on the primary replica of the second availability
group will show that seeding worked and that the LSNs were synchronized.


The output shown in sys.dm_hadr_automatic_seeding on the primary replica of
the first availability group will show a current_state of COMPLETED.
Seeding also has different behavior with distributed availability groups. For
seeding to begin on the second replica, you must issue the command ALTER
AVAILABILITY GROUP [AGName] GRANT CREATE ANY DATABASE command on the replica.
Although this condition is still true of any secondary replica that
participates in the underlying availability group, the primary replica of the
second availability group already has the right permissions to allow seeding
to begin after it is added to the distributed availability group.

Source : https://docs.microsoft.com/en-us/sql/database-engine/availability-groups/windows/distributed-availability-groups?view=sql-server-ver15