What is the proration behavior when upgrading from a 30 day subscription to annual subscription?

[Timmy_EDOCODE]

We are running an app that is planned to have both a monthly 30 day subscription plan and an annual subscription plan. We want to handle when a user is part way through a monthly 30 day plan and decides to upgrade to an annual plan. The documentation describes what happens when upgrading from a 30 day plan to a different 30 day plan with the following formula:

plan1_cost + (plan2cost - plan1cost) * (cycle_days_left / total_cycle_days) = total_cost

This formula leads to a very low cost when plan2 is an annual plan. We could not find any documentation explaining what happens for our case.

Does someone know what the behavior is?

[Timmy_EDOCODE · Accepted solution]

After testing and going back and forth with Shopify customer support, it seems the remainder of the monthly plan is essentially credited and put against the annual plan. What the customer is charged when upgraded from monthly plan (plan1) to annual plan (plan2) is calculated by this formula:

(plan1_cost / plan1_total_cycle_days) x plan1_cycle_days_left = plan1_credit

plan2_cost - plan1_credit = charged_amount