Airtel — Plan conversion

I am using Airtel from more than 10yrs. Now i shifted to US on short notice and from here i don't see postpaid SIM is useful, still i need this airtel number active as i have registered this in all places. I need to convert this SIM from postpaid to pre-paid and airtel guys are saying to visit india and get it convert. This is totally harassment . My brother is also visiting airtel office with all documents and still they are not converting. We need some solution online for this on priority.