Credit memo with VAT

Hi all

I have the below situation and would like to understand what is the best approach.

1-  RI invoice has been created (from a 'S1' Sales order) on 01/10/17 with the  amount of 8000 +  1600 tax (20%) = 9600 gross amount

2- The VAT of 20% expired on 31/10/17 and the new rate is 21 % from 01/11/17

3- On 03/11/17 the finance team decided to raise a credit memo to apply against this invoice, but the new tax is 21%,  so the system calculate 8000 (taxable amount) + -1680 (Tax) = -9680 gross amount

What is the correct way to get the credit memo a mirror of the RI invoice with the same tax (if there is any).?