If you change just firmware for Z0=75 ohm it won't work for 75 ohm properly. It may show you proper resistance for zero reactance circuit (such as dummy loads and resistors). But if your circuit contains any inductor, capacitor or transmission line, you will get wrong measurement. Because it's Q-factor will be affected by 50 ohm hardware port, so you will get shifted resonance and different S11 and S21 reading.
