What is the rate of GST to be charged on advances received before the change in rate of tax if the supply is completed after the change in rate of tax?

If the invoices is also raised before the change in rate of tax then the old rate will be applicable even though the supply is complete after the change in rate of tax. Else, the new rate will be applicable.

(ICAI FAQ PUBLICATION 06-09-2017 : Time and Value of Supply -FAQ NO. 32)