Information about the scaling and sharding of Amazon DocumentDB

Yes you can migrate from an Instance based cluster to an Elastic cluster. I would look at the following procedure just using open source mongodump and mongo restore . . .

Use mongodump to dump the data from the source DocumentDB instance cluster:

mongodump --uri="mongodb://<username>:<password>@<source-cluster-endpoint>:27017/<database_name>"

Create the sharded database and collections in the target Amazon DocumentDB Elastic Cluster.

Migrate the indexes from the source cluster to the target Elastic Cluster.

Restore the data using mongorestore:

mongorestore --uri="mongodb://<username>:<password>@<target-cluster-endpoint>:27017/<database_name>" dump/

You can set up and run an AWS Database Migration Service (AWS DMS) task in Change Data Capture (CDC) mode if you want to replicate ongoing changes from the source cluster to the target Elastic Cluster.

Update your application to use the endpoint of the Amazon DocumentDB Elastic Cluster as the new database endpoint.

For more detailed information on the migration process, including prerequisites, index handling, and monitoring, please refer to the AWS Documentation:

https://docs.aws.amazon.com/documentdb/latest/developerguide/docdb-using-elastic-clusters.html

Hybrid Approach for Homogeneous Migration to an Amazon DocumentDB Elastic Cluster