An adjustment credit note was created on changing the subscription's PO number. 

Steps to replicate the Use-case:

• The customer has an active subscription (invoice paid already) with PO number set to 1122.                             
• Changed the subscription to change the plan and apply on the next renewal so this creates 
  a scheduled change. 
                              
• Bill 1 future renewal (1 year) which generates an invoice. Send the invoice to the                                

  customer pending payment. Once paid, record the invoice as paid which sets the next billing date from 2024 to 2025. In the timeline, it now shows that the next renewal date is still 2024 and the next billing date is now 2025 which is correct as I want the scheduled change to apply in 2024. 
  

• Update the PO number in the invoice to 3344, no issues. 
                                             

• Then, change the subscription to update just the PO number from 1122 to 3344, apply the changes immediately with the checkbox 'Apply prorated credits and charges' disabled. On submitting the form and the 'Preview of Changes' pop-up, it says 'there will be no immediate changes and an invoice will be generated upon renewal on 2025'. When clicked 'Confirm', it changes the current subscription's next billing date from 2025 back to 2024 with credits issued on the latest invoice.
                                 

Here, the flow could be observed as Schedule Subscription Changes —> Generate Advance Invoice —> Change PO Number at subscription level —> Scheduled Changes being removed with credits issued on the Advance invoice generated. 

If the merchant is on the 1st version of advance invoicing, then this is an expected behavior where the advance invoice generates credits if you change anything like the PO number or custom field could be anything. This problem was solved in the latest version of advance invoicing. 

If the merchant is in the latest version, then on clicking the bill future renewals option -Down they will get the schedule advance invoice link :