Currently I live in a townhouse with a low slope/flat roof in climate zone 4 with r15 foil faced (down) batt insulation in the 2×4 ceiling joists. No true attic access. limited space between the ceiling joists and roof support.

I’ve had the idea in the future to move the insulation to rigid foam boards above the roof to bring up the insulation value. Assuming right now there is a build up roof as the main assembly, how difficult would it be to transition to that type of insulation on the roof?

Would it make sense to replace the sheathing too to replace the current insulation? or take out the drywall ceiling and take from inside.