Share 'Retrofit insulation on the roof over cathedral ceiling'

I have an under-insulated roof above a "cathedral ceiling".  The shingles need to be replaced this year.  It occurred to me that this could be an opportunity to insulate from above. Can it make sense to insulate above the plywood roof deck? Or to remove the plywood roof deck, add insulation, and re-install the plywood roof deck on top of that?  Accessing the insulation from inside would require r…