I think there are different situations here:
1. The POH has US gallons of fuel, pounds of mass and pound-inches of leverarm. It would be simplest to be able to enter these in the aircraft profile straight without any conversions (to avoid mistakes), but be able to tell if the pump has refueled enough liters and put in people weighing kilograms. I will call this scenario "convert to display units".
2. The plane displays speeds in mph or km/h, not necessarily matching the POH. Here you want to keep displaying these units. I will call this "change display units".
3. Any combination of 1 and 2.
So for any profile, you would need basically a table of what you want converted to baseline display units and what not. A little heavy I guess.