I think it is a terrible idea to go from 2 norco's a day to a 12.5mcg/hr fentanyl patch. 20 mgs hydrocodone a day isn't even equivalent to half of the opioids you'd be receiving from the fentanyl patch. Perhaps try an extended release formulation of morphine or oxycodone, even hydromorphine, before switching to fentanyl. Fentanyl is basically the last step you can take in pain management, there isn't really anywhere to go up from there (besides increasing the fent dosage), so before you leap onto such a powerful medication, try alternatives.
This is only my opinion, but any doc who would recommend ANY dose of fentanyl for someone taking 2 norco's a day should probably lose their controlled-prescribing license.
My statements are to be considered opinions only. I am not a medical professional.