Just remember that water _usually_ flows downhill. A properly flashed
pitch change will take into account the local climate, the
insulation/ventilation, the pitch and the materials you use. In a
nutshell, you'd tear off the lowest area of the sloped roof, install
the new flat roofing material up to where you stopped tearing off, then
install new or dependable sloped roofing material over the new flat
material starting just a few inches above the pitch change. Simple, no?