Sales Order Invoicing Schedule Revision After Invoice Issued

We will occasionally need to change the invoicing schedule on a given sales order after the customer has already been billed (and revenue recognized).  An example is:

1. Customer billed $12K annually.
2. Customer billed $12K on x date.
3. Three months after x date, Customer switches to monthly billing.
4. We credit $9K to original invoice via credit memo and then add invoicing schedule for remaining $1K/month for 9 months.

Problem is that the sales order value is then overstated by $9K as it won't reflect the $9K credit applied against the original $12K invoice on the sales order screen (it will show a total value of $21K).  I've tried to issue a negative invoice instead of the credit memo, but NetSuite won't let me - is there a way to do this that will get the sales order value correct and show the credit somehow?