post paid to pre paid

Location/place: Pune

Name of company/service: Airtel

I had a airtel pre-paid card. To go to J&K I got it converted to post paid.The sales representative there did not disclose even after querying on the terms/conditions to convert back to pre-paid. After coming back I now want to convert to pre-paid, the relationship officer says I need to complete 3 months to convert back to prepaid. Want to know if there are any such conditions laid out by TRAI?