> ## Documentation Index
> Fetch the complete documentation index at: https://docs.slm.m1amdocs.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Modifying AutoBills

`AutoBill.modify` may be used to edit, add, remove, or exchange a Product on the existing AutoBill, or to replace the existing Billing Plan with a new Billing Plan.

`AutoBill.modify` is designed to:

* Work with AutoBills that have any number of AutoBill Items.
* Allow the addition, removal or replacement of any AutoBill Items, in a single call.
* Work with Campaigns.
* Retain historic AutoBill data (such as payment history and Product choice evolution) while making changes to existing AutoBills.
* Generate a prorated net charge or refund for the combined modification activity. This charge or refund will appear through the API and GUI with other Transactions from this AutoBill.
* Modify the quantity with or without proration—if, in the middle of a billing cycle you change the quantity of an item, you can choose whether or not to prorate.
* Keep the AutoBill in its original state if any aspect of the call, including the modification charge or refund, fails.

**Note:** AutoBills containing Rated Products may not be modified.

The `AutoBill.modify` call allows you to change the existing Products or Billing Plan for an AutoBill, without losing the history of the subscription.

Do not use the `AutoBill.modify` call to change anything other than the Products or Billing Plan for an AutoBill.

* **[Prorating Modification-Based Price Changes](../topics/Prorating_Modification_B)**

* **[Changing Products for an AutoBill](../topics/Changing_Products_for_an)**

* **[Changing the Billing Plan for an AutoBill](../topics/Changing_the_Billing_Pla)**

* **[Changing both Products and Billing Plan in a Single Call](../topics/Changing_both_Products_a)**

* **[Viewing AutoBill Changes](../topics/Viewing_AutoBill_Changes)**

* **[Continuous Billing during the Retry Period](../topics/Continuous_Billing_durin)**

* **[Advance and Arrears Billing Option for AutoBills](../topics/Advance_and_Arrears_Bill)**

**Parent topic:**[Working with AutoBills](../topics/ProgGuide_AutoBills)


This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.