An employee's Employment Type can be changed within their Employee Profile.
Settings > Employees > Edit, on the individual employee
Change the Employment Type as required
The Entitlement values of an employee should also be reviewed when changing the Employment Type as this can have an effect of their entitled leave.
For Casual, make sure the entitled Annual Leave and Personal Leave is set to 0.
For Full time or Part time staff make sure the entitled Annual Leave and Personal Leave is set, typically 4 weeks for Annual Leave and 2 weeks for Personal Leave.
If the employee has a leave balance and is changing to Casual, then leave will need to be paid out.
This can be done on the payslip by adding a leave payment to bring the balance to 0.
For Annual leave:
On the payslip go to Add Leave > Annual > Cash Out
Fill the Hours field with the current Annual Leave balance.
For Personal Leave:
On the payslip go to Add Leave > Personal
Change the Class to Bonus, the Rate to 0, because Personal Leave is forfeited
Fill the Hours field with the current Personal Leave balance.