That’s called “rebasing” and it needn’t be done for every currency out there. You can just declare “every penny in circulation is now worth a nickel.” It doesn’t change the price of anything, as your total at the till can still be $4.34, but you’d have to pay a total of 4.35 and every time you hand over that rust-colored Lincoln coin, it counts as .05.
There’s no need for “old dollars” and “new dollars”.