The License and Roles required to perform Viva Engage(Yammer) Migration are as below :

1. Microsoft 365/Office 365(E3 or E5) License is required for Source & Target Connector Accounts.
2. Yammer Enterprise License is required for both Source & Target Connector accounts.
3. Yammer Administrator + User Administrator role is required for both Source & Target Connector accounts.
4. Source Connector Account should be a Member of the "Viva Engage(Yammer) Private Groups" that you chose to migrate.