Hi Sammy,
As you see, you can't change the tax rate on the fly for credit notes (or invoices). What you need to do is Copy your Tax Group to a new code and maintain the old GST rate (6%) there. When you need to issue a credit note based on the old 6% rate, change the tax group on the credit note to the other tax group code that uses 6%.
For a list of changes to how to change a tax rate, see this tech tip by clicking
here.