Airtel — not allowing to move from postpaid to prepaid

I am a airtel postpaid customer for 2 months.
And need to covert to prepaid but when i went to store they said minimum time for conversion is 5 months, that is till 5 months i have to use in postpaid, but my usage is very less and i have to pay minimum rs.230 per month. This is waste of my money, is there such a rule?. If there it should be changes. A customer should have the right to change from prepaid to postpaid and postpaid to prepaid at any time without any specific time limit to stay in one service like postpaid or prepaid.