They will change your box at any point of time. Don't worry about the deadline TataSky had set.
The only problem would be, at some point of time soon, they will move some of the channels (Hindi?) that you might be watching to MPEG4 transmission, after which you will have to force yourself to change your set top box.
I suggest, if you have a good offer, upgrade to HD or HD PVR or even 4K box -- they will be near future proof.