In this tutorial, I will go over how to change a particular product from monthly billing to annual.

WHMCS - Change the billing for a product from monthly to annually:

1. Input the annual price in the 'Recurring Amount' field in the 'Products/Services' tab.
2. Set the 'Billing Cycle' drop-down field to 'Annually'.
3. Click the 'Save Changes' button.

Cancel any previous monthly invoices.

If a monthly invoice has already been created, you will have to cancel the invoice and generate a new invoice to reflect the new annual billing cycle.

1. Click on the 'Invoices' tab and cancel the monthly invoice.
2. Go to the 'Products/Services' tab and set the 'Next Due Date' back one day and click the 'Save Changes' button. Doing this will allow WHMCS  to generate the due invoices again.
3. Click on the 'Summary' tab.
4. Click the 'Generate Due Invoices' option.
5. If you didn't select the email invoices option when clicking the 'Generate Due Invoices' option, you may want to manually email the newly created annual invoices by clicking on the 'Invoices' tab, then click the invoice, and click the 'Send Email' option.

Note: If your customer is overdue on several invoices, you may be able to account for this in the new annual invoice by change the next due date, after all the other steps have been completed.