Migration to ATLAS from an on-prem 5.0.3 single instance no replicasets MongoDB?

What are my options to do this without causing no or minimum downtime?

Hi @Mike_Frost

To do this with minimal downtime you will need to convert the existing database into a single node replica-set and then use live migrate.

You can do this manually or use Cloud Manager or Ops Manager to assist in this process. Then you can use the Live Migrate method that best suits your organisation.