The primary function of the aux fan is to bring down high side pressure when it gets dangerously high. This is to save the a/c system from excessive and damaging high side pressure.

Some of the high pressure switches mounted to the r/d are in actuality temperature switches. Pressure directly correlates with pressure. If it is a temp instead of a pressure switch, you can change it without losing refrigerant.

Regardless of whether it is a temp or pressure switch I would not change it until I determined that it is bad. Get a wiring diagram for your car and it will probably indicate the pressure at which it trips. If it is temperature, you need to look at the scales on a high side gauge to correlate this with pressure. Then simply monitor the high side gauge with no ram air and the engine reved to about 1,500 RPM. The high side pressure will increase because there is little air flow. When the pressure reaches specification if the switch does not actuate then replace it.

Good luck,