post paid to prepiad

I wanted to change mobile number billing from postpaid to prepaid, 4 months back I started using postpaid , I went to Yelahanka idea store in Bangalore. the store person is saying after being postpaid for 6 months you can transfer.

But I called customer care and enquired they are saying 3 months. same thing I said that to idea customer care person saying 3 months still he is not willing to accept or check what is correct.
Instead of that he is saying me to go to Richmond road idea store . why i need to go to Richmond Road which is 15 km from here.

And even I went to Malleswaram idea store and I made to talk the store person with customer care and customer care people said 3 months and still the store person is sticking to 6 months and she told me to come after completion of 6 months.

I am confused with different answer from these idea people and want to know the real procedure which government allows and eligibility criteria to migration from postpaid to prepaid, and even sent mail to
[email protected] nothing much happened the email reference id is E1-33592762314.

kindly show me the way to get migration from postpaid to prepaid.