[TheGeminon]

Devaluing the new currency by adding lesser metals will also devalue existing currency that is "pure" as you aren't able to trust the value of the currency anymore, so the value of the existing pool of money will drop.

Its at a smaller scale, but it can be seen with counterfeit currency today. Cash-heavy businesses have to absorb whatever amount of counterfeits they accept, so they are really valuing your dollar at $0.99 if they might have to throw it out.