I just recently upgraded three boxes with new WDxxPURZ hard drives, and each required a reach out to FiOS. I learned from the first box, which took many hours!
I learned this from this TiVo community site ... you log into the FiOS website, use the FiOS chat function, get a FiOS TV support agent via Chat and ask them: "Hi. I need to manually validate a cable card". I then gave them the Cablecard Serial Number, Cablecard ID, Host ID, and Data ID. The first experience via chat and calling support was a total nightmare, many hours, but after that experience I learned and followed the above, it then worked perfectly thru chat. Again, the key, I copy/pasted: "Hi. I need to manually validate a cable card", and then it was just a few minutes each time. I hardest part was the wait time for a chat agent.
Any change of the hard drive, or a swap of the cable card to a new box, requires a repairing of the cable card. You will not be able to get any premium channels (HBO, ...) or copy protected channel (Fox News, FX, ...) until the re-pairing.
You know that you have this issue if you go to: Settings ...Remote, CableCARD, & Devices ...CableCARD Decoder ...CableCARD Installation ...CableCARD Menu ... Conditional Access.
Then look at the 4th line, at "Val" value. If there's a "?" then it's not paired yet. As an example, one on my boxes now shows: "Val: V 0x00".
If there's a "?" you're not paired.