Updating employee from Casual to Salary

I have an employee that is moving from casual (timesheets) to a Full Time employee on salary. 

When I try to update her employee card, as soon as I change the Pay Basis to "Salary" the Wage pay Item of "Base Hourly" disappears and changes to "Base Salary".  The result is that I get the below message"

"One or more payroll categories cannot be deselected as they have been used on recorded timesheets" 

I understand that it cant be deselected or deleted but this is somethign that is happening automatically.  Please help