Once you register a customer for functions designated as GSTINC, you cannot subsequently change that registrant's Tax Authority on the Register a customer - Attendee tab until you un-register the customer from the GSTINC functions.
If you try to change the Tax Authority before un-registering, an error message will be displayed and you will not be allowed to make the change. After un-registering the customer, you can change the Tax Authority, and then re-register the customer.