Will I be required to pay GST at the new GST rate on Debtors outstanding as on the date of change in the rate of tax?

If the supply was complete for such outstanding balances and the invoice is also issued before change in rate of tax then old rate will be applicable. Else the new rate will be applicable.

(ICAI FAQ PUBLICATION 06-09-2017 : Time and Value of Supply -FAQ NO. 36)