nah, all you are doing with the half power switch is running the output tubes in triode mode instead of pentode mode, that's how it drops the wattage down. the impedance switch should remain at 8 ohms if you have an 8 ohm speaker.
Actually im pretty sure the "half power switch" does remove the two outer tubes but the amp creates a dummy load so the amp "thinks" it's still using all the power tubes and the impedence is unaffected. At least thats how Nik wired the earlier "half power" switch.
A pentode/triode works as well but I think this is more intended for 50 watt (two power tube) amps. Triode/Pentode is going to have a mushier sound in triode where removing the two outer tubes (on 4 tube amp) should leave the tone relatively unchanged.
Either way, no the impedance does not need to be changed unless you manually remove the two outer tubes.