Don't confuse , i can give the exact answer,
as u said , when nmos and pmos are interchanged in the basic cmos inverter , the output swing will be from vdd-vth to vth.
reason: take nmos pass transistor , u give 1 terminal to vdd and the gate terminal (vgs) to vdd. now u can't get vdd at the other(sourse) terminal of nmos , this is bcoz when the sourse is charged to vdd-vth then the effective vgs = (vdd-(vdd-vt)) => vgs = vt. now u can't furthur charge the sourse as it goes into the cutoff if the voltage is less than vt for the nmos transistor. so nmos transistor is off(cutoff) and the output is vdd-vt.
apply the same for pmos , now analyse the cmos inverter gate , u will find the answer.