Hi,
I have just had my set top box replaced with a modem (nice shiny 250!) as I no longer had the TV and it was taking up too much space. I didn't get charged for this, I just rang up and asked for it to be swapped and they did.
And as a bonus I can now get 2Mb as my old STB only went to 1Mb
So I don't see why you would get charged..