It seems like most everything manufacturing wise already does both.

Obviously changing the road signs would be the largest cost I'd think, but I feel like that could be done over time, since cars have both MPH and KPH speedometers already. Maybe that would be too messy though.

Construction materials making the conversion? Parcels/Land area?

I think it makes sense to change it, but I agree that if the cost is too much, there is no reason to spend additional money on it.