DTH Portability is NOT practically possible at this moment. Since different service providers use different compresson technology and Set Top Box.
Its not like Mobile Number Portability where its possible to replace only SIM. You buy your own choice of phone from open market. This is not the case with DTH. Every providers has different box and technology. Some uses Mpeg2, some Mpeg4.
However, they can sell SmartCard for Rs 500 or so separately but again you have to buy STB with CAM Slot which is not cheap and your previous providers STB will be of no use. So its not economical.
Thus, its not technically, practically and economically possible at this moment.