Vodafone India — postpaid to prepaid

I had applied for postpaid to prepaid for my number [protected]. They assured me to be done within week. For that i had already paid advance rs. Around 350.
Then on 19 th, 21 th august i called customer care they assured to do it within 24 hrs. But till date 31 st they have not done anything.
Moreover they generated my next bill and said you have to pay this.. But why should i pay for that its your fault that you could not do it within week.
As per rule it should be done within 2 working days.. I had aleardy paid for 8 more days..
Kindly do the needful.