I agree with C-H in that we didn't seem to cover this well. It looks as though I missed this thread when it was posted as well
-- My apologies.
Wouldn't the change in frequency affect the impedence of any circuit that is not purely resistive?
It sure will.
Inductive reactance = 2 * pi * f * L
Capacitive reactance = 1 / (2 * pi * f * C)
So XL or Xc will be correspondingly 20% higher or lower. If the power factor of the circuit is fairly low, then that could have a considerable effect on the overall impedance.