You have a requirement to enhance some of the Account Hierarchies, but you are not able to find migration mode Enhance Already Existing Records
Reproducing the Issue
1. Go to Business Configuration work center.
2. Go to Overview view.
3. Search and open the activity Migration of Account Hierarchies.
4. Select Migrate Account Hierarchies using migration tool
The system works as designed.
In current release, Enhancing Account Hierarchies is not possible. Ideally while creating the hierarchy the whole hierarchy with accounts should be migrated. If an hierarchy already has an account then more accounts can't be added to that.
As a work around, you can go to Customer work center, select an Account and maintain Parent Account.
This feature will be available in one of the future releases of SAP Cloud for Customer.
KBA , SRD-CC , Cross Components , How To