Receivable under service tax where tax is payable on receipt basis.

Receivables for service done and bill raised under service tax act before 30th June.
The individuals with low TO are allowed to remit tax on receipt basis. What is transition provision on this?
Is tax payable at 15% as per bill or 18%.?
When is the tax on the bills outstanding on 30 June is due for remittance?
Please provide circular / clarifications if any.
Thanks.