To change the tax rates in Withholding Tax Codes or create new?

Hi

TDS rates have changed w.e.f 1.6.2007 for certain sections. Our clients wants to change the rates with a certain validity dates. There are certain bills to be processed yet for dates before the validity dates of the new tax rates. Is it advisable to change the rates in existing taxe codes or should we create new tax codes for the new rates. Pls advise

Regards

Sanil Bhandari