Different invoice terms for different users

[freedombi]

Currently, whmcs generates invoices on the due date, and a database trigger to add 21 to the due date on the invoice when it's created. This would create, for example, an invoice with an invoice date of Nov. 1, a due date of Nov 21, and for services from Nov 1 to Nov 30. This is to match how things were done before we started using WHMCS, that invoices for the current month go out on the 1st and are due on the 21st.

 

What I'd like to do is to find a way to have it continue to do this for current customers, but for new customers, to bill in advance. eg, if I want it to generate the invoice 21 days in advance, it would be generated on October 11, due Nov 1, and the services would say Nov 1 to Nov 30.

 

Adding a custom field would be the easy part of this. But how could I make it generate invoices one way for some clients, and another way for other clients?