Would you like it sensor-specific, model-specific, or as a simplest way - system specific (that means an option added in Config menu)?
System specific would break the concept of sensors defining what kind of parameters they know and how the unit is displayed. This is one of the strengths of the Jeti system compared to other vendors.
Everything else but sensor-specific is a bad idea, here's an example:
Vario climb/fall rates is displayed in m/s, the speed is shown in m/s also. I definitely would not want a system specific setting to change m/s to km/h in general, because for the Vario the m/s is needed. Such settings belong to the sensor.