It is often difficult to safely and reliably switch between opioid medications. Although there are 'morphine equivalence' charts and 'opioid equianalgesic charts', these often can lead us down the wrong path as opioid dosing is highly individualized. Conversion charts don't, and can't, take into account opioid tolerance, individual metabolism and how someone will respond to a different opioid agent (i.e. cross tolerance).
Switching Between Opioids
It is always a good idea to be extremely conservative when switching between opioids to prevent the risk of overdose. You can always treat symptoms of withdrawal by adding on breakthrough opioid medication. Overdose is much more serious. This is especially true of fentanyl patches. The prescribing information for fentanyl patches strongly advises against using any kind of conversion chart. Per the package insert of Duragesic (fentanyl):
"Do not use [morphine equivalence charts] to convert from DURAGESIC to other therapies because conversions to DURAGESIC are conservative. Converting from DURAGESIC will overestimate the dose of the new agent."
In other words, using a equivalence chart to convert from fentanyl to another opioid agent could potentially lead to overdose, and should not be done.
Since conversion charts should not be used, the general recommendation for converting from fentanyl to oral opioids is simply to titrate the dose of the new analgesic until adequate pain relief is attained.
This obviously leaves a lot of room for interpretation and this is where your doctor and pharmacist come in. They have a much better understanding of your current medical situation and should be the ones to provide you with an appropriate recommendation. As mentioned above, opioid dosing is highly individualized and it wouldn't be appropriate for us to give you a specific recommendation. We can however, give some general information for reference only.
In your question you state that you are stopping fentanyl 50 mcg/hr and instead going to be increasing your dose of Norco by 40 mg per day (to a total of 80 mg per day). This does appear to be an appropriately conservative dose.
Morphine Equivalents To Fentanyl
Below is a chart that practitioners often use when converting from morphine (or morphine equivalents) to fentanyl to give us a frame of reference.
Again, this chart should not be used to convert FROM fentanyl TO an oral opioid but it gives you a general frame of reference. Hydrocodone is most commonly converted to morphine in a 1:1 ratio, meaning 1 mg of morphine is equal to 1 mg hydrocodone so you can theoretically read the left side of the chart as 'hydrocodone dose'. You are DISCONTINUING 50 mcg of fentanyl. The recommendation your doctor is giving in regard to your Norco dose change appears appropriately conservative.
It is important to know that once a fentanyl patch is removed, 17 hours or more are required for a 50% decrease in fentanyl concentrations. Due to this many practitioners advise to wait at least 12 to 24 before initiating any new opioids. When new opioids or doses are introduced, it is important to dose very carefully to avoid potential overdose.