What is the resulting Voltage Transfer characteristics of the modified CMOS-inverter circuit if NMOS and PMOS are interchanged?
Switch them and they're both acting as drain followers (like an emitter follower).  The circuit will stop inverting, but the output will not go closer to either rail than Vgs(th) unless pulled externally.  If the load has a significant bias current, it will be pulled to one rail by the bias current and driven within Vgs(th) of the other rail by the buffer.