I just upgraded from a Tivo HD to a Premiere, so I had basically the same situation. I moved the cablecard and at first it seemed fine. Then periodically, even when I was watching Netflix, I'd get an error screen popping up about the card not being activated. Then a day or two later the pay channels stopped working.
To get to Comcast's CableCard people, call 1-877-405-2298. Make sure you have your CableCard info ready (you can find it somewhere down in the Settings menus)....they'll need various numbers like the CableCard ID and such. It took me about 10 minutes to get it done. I've read that it's best to do it during the week though, that weekend they tend to want to send a tech out.