Yes, you can raise a supplementary invoice / debit note to recover the additional tax from the customer.
(ICAI FAQ PUBLICATION 06-09-2017 : Time and Value of Supply -FAQ NO. 37)
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)
If 95% of the work is complete before the change in rate of tax but invoice can be raised only after completion of supply, then what is the rate of tax to be applied?
Assuming the supply is completed after the change in rate of tax, new rate will apply.
(ICAI FAQ PUBLICATION 06-09-2017 : Time and Value of Supply -FAQ NO. 33)
How do we determine the rate of tax in case there is change in the tax rates?
Three important events need to be considered – Date of raising invoice, receipt of payment and completion of supply. If any of the two events occur before the change in rate of tax then the old rate will apply else the new rate will apply.
(ICAI FAQ PUBLICATION 06-09-2017 : Time and Value of Supply -FAQ NO. 30)