This update makes it easier to upsell non-recurring products when the subscription start date aligns with the next invoice date. The change improves the process by separating constraint logic, allowing for greater flexibility and preventing potential errors related to zero prorata ratios during the upsell confirmation.

Make easy for module that extend sale subscription to allow for some specific case to upsell subscription with start_date = next_invoice

Forward-Port-Of: odoo/enterprise#112730