When you use the Change subscription option through the admin console or the APIs, you will have an option to choose "Apply prorated credits and charges" that lets you provide credits to your customer for the unused period. When you Apply changes the prorated credits gets applied and an invoice gets generated for the upgraded plan.
Note: 1. You can also choose to either apply the changes immediately (the charges may be prorated then), on the next renewal, or even schedule it for a particular day. 2. Any scheduled change will be overridden if a new change is scheduled. 3. APIs carrying parameters for proration and unbilled charges will not work in the case of scheduled changes
To know more on where the change Subscription can be accessed from and how to apply changes refer to this document.
Here is more information on proration. Here is more on proration on subscription change in specific
Warning: The next billing date does not change when you change a subscription from plan A to plan B with the same billing frequency.
If the plan frequency is different then the customers will be charged for the new plan and the billing frequency will be set accordingly.
If you need to change only the next billing date use the “change the next billing date”(Subscriptions details page > Action panel to the right) date and prorate the charges if applicable. Here’s a video for reference.
Related articles and Documentation
How can I add or deduct promotional credits for customers?
Why isn’t the promotional credits applying to my customer’s invoice?