Vodafone India — wanted to convert postpaid to prepaid

Last month I visited Vodafone store, I told them that this sim is my main sim and am facing network problem in this. He told me that you haven't recharged it, and you have passed your trace period also. Now there is only one option to get back your same no. You have to take postpaid connection of same number...some people told that I was cheated at store, it is plan to fulfill their targets. Actually at that time there were approx. 20 ruppee in my prepaid account. I want to get to know whether the above procedure was legal or I was cheated? And I also want to convert it back to prepaid, being a students plan of 299 is too costly for me.