Airtel — poor customer support. excessive bill charged for postpaid to prepaid conversion

I've been using airtel postpaid sim for more than a year. Because of the excessive bill charged every month inspire of not using the sim on a regular basis i decided to change my sim from postpaid to prepaid. When i enquired about the same at the airtel office coimbatore i was asked to pay my regular postpaid bill of rs. 300 along with an extra 200 for using the sim for the next 4 days. It didn't stop there!!! I was also asked to pay rs. 399 for converting my postpaid to prepaid. When other networks charge a porting fee of just rs.50 or sometimes even do it for free of cost, this kind of a service by airtel is completely disgusting.. Any network that wishes to retain it's customer for a long term would never impose such high charges for just a post paid to prepaid conversion!!! What can be a better answer you can give me for this??!!!