How to update Start and End Dates for Credit Memo with Invoice Rules?

 Hello,

Can you please advice on the below scenario?

1) We issued an AR Invoice to a customer with "In Advance" Invoice Rules. Deferred Daily Rule for the period 1-May-11 to 30-Apr-12 is attached to the Invoice. The GL Date on the Invoice is Apr-11

2) The Customer now cancelled the subscription for the product/service provided.He cancelled the subscription only for the last 4 months, i.e., from 1-Jan-12 to 30-Apr-12

3) We issued a CM for the invoice. The CM inherits the rules from the Invoice and hence is taking the rules from 1-May-11 to 30-Apr-12.

4) We would like to change the dates on CM to 1-Jan-12 to 30-Apr-12.