Yes. Any employee who changes their status to that of a principal will need to have their insurance upgraded to principal level. This will require the difference in the premium between an employee and a principal to be paid.
This is calculated on a pro rata basis taking into account what has already been paid as an employee, provided the employee levy has been paid by you. The reverse applies if one of the principals retires, and is then employed as a consultant and wishes to change his or her practising certificate to an employee certificate.