Atlas displays a walk-through screen that collects information required to connect to the source cluster.
Enter the hostname and port of the primary member of the source cluster into the provided text box. For example, mongoPrimary.example.net:27017.
If the source cluster enforces authentication, enter a username and password into the provided text boxes. The username must correspond to a MongoDB user on the admin database with the following permissions:
readAnyDatabase on the admin database.
read on the local database.
If the source cluster uses TLS/SSL, toggle the SSL button.
If the source replica set uses TLS/SSL and is not using a public Certificate Authority (CA), copy the contents of the source cluster’s CA file into the provided text box.
At a glance, I think the requirement for a user with readAnyDatabase on the admin database will prevent live migration from the Compose ‘mongodb classic’ hosting because you don’t have access to the admin database.
I suspect this will work if you migrated to the new mongodb+ deployments on compose first, because they have an admin database you can access, but I’d speak to Compose about that. Not sure you’d be able to do a zero-downtime live migration from mongodb classic to mongodb+ anyway (and there’s slight additional cost involved because you pay more for 1st GB, pay extra for oplog etc with mongodb+)…
If you’re already on the new MongoDB+ deployments at compose then you’re probably providing the wrong user credentials.