Suppose, for example, 10 coins of the old ten-yen currency could be changed into 1 new ten-yen coin, the value of the old ten-yen coin becomes only one tenth, which means melted copper ingots may have more value than the new copper coins.
たとえば10枚の10円玉を新10円一枚に取り替えられるとすれば、旧10円は1円の価値でしかなくなるので銅にしたほうが得、という理屈だ。