Composition scheme for payment of service tax

I am service provider under works contract servicve and opted to pay service tax under composition scheme. I completed a work and raised invoice for the same in 2007 and received consideration against the bill in 2008. However in 2007 the rate was 2% but in 2008 it became 4%, so in absence of POT, rules2011 at that point of time , which rules should i follow to ascertain that i should pay tax @ 2%.