The question is: you can't fire him, you ought to transfer him. Which, at its turn, returns us back to the problem of transfering soldiers' nationality as if it's nothing.
Had the artillery costs only the 5 oil as upkeep, you would have to be right.
you could also say the upkeep is the upkeep to keep then clean, store them , storage employees? I still think of trading units as trading the arms they used. Storing weapons isn't free either just like storing any other goods. In the industry knowns as storage risks and storage costs.