I did not really get your question.
Set-top-box generally, means a STB (the box) and a smartcard (digi-card). So, when you change both of them, it means, you are completely changing the Set-top-box.
Are you trying to upgrading to HD / HD PVR and you don't want to use the old set-top-box and you don't want to pay the installation fee of the new Set-top-box?
If that is the case, I really double TataSky would allow you changing that without paying-installation fee. However, if you escalate the call to Senior level, it can be done. (In my account, I have two in-active boxs (One SD, one SD PVR) and one active box (HD PVR). I did not pay installation feel when I upgraded to HD PVR box.