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# Prorating Modification-Based Price Changes

When making the `modify` call, you must specify an effective date, which can be the next billing date, or the current date. If you elect to set the `billProratedPeriod` input variable to `true`, Subscribe will calculate any prorated charges or refunds as of the effective date. (The net charge or refund is calculated based on the difference between your customer’s current AutoBill charge, and the modified AutoBill’s charge, as of the effective date.)

If you want the change to take effect immediately, specify an effective date of `TODAY`, and issue a pro-rated charge for the partial Billing Period that your customer will have entitlement to the new Products.

For example, if a customer is billed on the first of every month, and on the 7th day of a 30-day month the customer changes from a $10/month product to a $15/month product, the month’s charges will be calculated as follows:

(new plan cost - old plan cost) \* (number of days on new plan / number of days in period) =

($15 - $10) \* (24 / 30) = \$4

**Note:** AutoBills containing license based rated products can be changed at any time, with the same proration options as rated products.

\$4 is the cost of the change today, and will be charged to the customer immediately. (Set the *billProratedPeriod* flag of the `AutoBill.modify` method to `false` to skip this charge, if desired). If you are performing a downgrade, the cost will be negative, and will be immediately refunded.

The customer’s next bill would then be on the first of the next month for \$15.

Using the `AutoBill.modify` call to change the customer’s Billing Plan will always reset the billing date to *today*. Any surcharges or refunds will be calculated appropriately.

**Note:** Use the *dryrun* input parameter for the `AutoBill.modify` call to return any charges or credits that would be levied as a result of the change, without modifying the AutoBill. This allows you to show your customers a preview of changes to their subscription, without saving the change.

**Parent topic:**[Modifying AutoBills](../topics/Modifying_AutoBills)


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