It depends on the circuitry used to drop the power to "half". Some amps do remove two of the 4 power tubes from the circuit [often by lifting the cathodes from ground] and that will require an impedence change. Others use lower B+ voltages, which does not require a change. Others again use triode/pentode switching on the output tubes, whcih does not require a change.
Which amp are you thinking of?
My Carvin Legacy has a half power switch. Can I just throw the switch without worrying about the speaker impedance?
Most of the boutique amps are overbuilt....so some mismatch isn't an issue. Rick