Advance amount is always subject to final adjustment at the time of issuance of invoice/completion of service irrespective of the fact whether it is service tax era or GST era. Advance itself cannot be treated as consideration without providing service/supply of service. See definition of 'consideration' under Section 2(31) of CGST Act.
"(31) “consideration” in relation to the supply of goods or services or both includes -
(a) any payment made or to be made, whether in money or otherwise, in respect of, in response to, or for the inducement of, the supply of goods or services or both, whether by the recipient or by any other person but shall not include any subsidy given by the Central Government or a State Government;
(b) the monetary value of any act or forbearance, in respect of, in response to, or for the inducement of, the supply of goods or services or both, whether by the recipient or by any other person but shall not include any subsidy given by the Central Government or a State Government :
Provided that a deposit given in respect of the supply of goods or services or both shall not be considered as payment made for such supply unless the supplier applies such deposit as consideration for the said supply;
Liability of GST arises on this advance amount but this advance will become consideration only at the time of supply of service/issuance of invoice.