Add a Secondary Domain for the newly created Primary Domain.(Primary domain cannot be changed). Change is made from the CMP console for the CASB tenant.
This will not impact the functionality, All domains are considered internal domain\users in CloudSOC.
There is no difference in how a primary domain is treated versus a secondary domain.
Talk to Broadcom sales to create a new CloudSOC Tenant with the new Primary Domain and delete the original tenant.
In the specific case the primary domain cannot be considered a internal domain from CASB perspective.