The government's current change, in the way paper money looks, is to deter fraud/counterfitting.
I agree we are told they are legal tender for all debts public and private, so I wonder how this would work.
But if a distinguishable new currency were introduced for domestic use only, and by law, not exchangable with the old-design currency circulating only outside the US, both could be "legal tender", just not exchangable without explaining where one got the currency they had.
Then there is the issue of exchange rate. Presumably they'd be $1 to $1, but then I assumed the Fed would never induce inflation either. Agitator in post#8 seems to suggest otherwise.