Cannot change invoice type to prepayment

If you create a Standard invoice in the invoice workbench and save the record (no lines or distributions entered), then you realize that invoice type should have been prepayment but now you cannot update it.

It appears that once you save the record you can no longer change the invoice type to prepayment. You can change to other invoice types, just not prepayment. The same is true for invoices created through the interface.

Is this intended functionality? If so, why?