Jet allows domestic upgrades even on award tkts
I've upgraded award tkts at airport several times on 9W as long as they have the right class. On MAA-DEL it costs about Rs. 2000/- for a Platinum member (absolutely worth the cost for the 2.5 hrs flt).
International upgrades are available using miles but you need to do so upto 3 days before by faxing the JP center. I have not tried cash upgrades from Y to J or J to F on international segments.