In my opinion, the excess payment of service tax can be adjusted in the subsequent month or quarter and change of method does not effect adjustment of excess service tax payment. The relevant section is:
Rule 6 of Service Tax Rules, 1994:
Where an assessee has paid to the credit of Central Government any amount in excess of the amount required to be paid towards Service Tax liability for a month or quarter, he may adjust such excess amount paid by him against his Service Tax liability for the succeeding month or quarter.
The Conditions for Self-adjustment of Excess amount of Service Tax paid :
(i) Excess Service Tax paid is on account of reasons not involving interpretation of law, taxability, classification, valuation or applicability of any exemption notification,
(ii) Excess Service Tax amount paid by an assessee having Centralised Registration [Rule 4(2), Service Tax Rules, 1994. ] on account of delayed receipt of details of payments towards Taxable services may be adjusted without monetary limit.
(iii) In cases other than specified in clause (ii) above, the excess amount of Service Tax paid may be adjusted with a monetary limit of1,00,000/- (one lakh) for a relevant month or quarter.
(iv) The details and reasons for such adjustment shall be intimated to the jurisdictional Superintendent of Central Excise within a period of 15(fifteen) days from the date of such adjustment.