The US has last partially defaulted on its currency in 1971, surely a change of the design is possible too.
It's changed plenty of times [1].
[1] https://en.wikipedia.org/wiki/History_of_the_United_States_d...
(I know, I know, it's not actually that big a deal. The drawers have removable inserts, but you get the point of it.)
(actually, on second though, automatic bill readers in vending machines, etc. would need a big retrofitting, that's probably much more a big deal).
But they had suspended convertibility of dollars into gold every so often before that. (And, of course, the dollar wasn't always about gold. They also experimented with silver and bimetalism.)
They suspended redemptions of the certificated amount. You could not own gold until 1933 from 1974, but excluding of that, you can always convert your money into gold outside of the government if you want.