Yes exactly. As long as there is a mutual agreement between everyone, it is not impossible to even create new currency to be used or change the financial system in which it won't be dependent on USD. In regards to changinf currencies, we are all aware that there have been mamy issues involved with USD even from before, and there are so many countries who are already waiting to be next reserved currency.
The currency alone is easy, but the agreement is the one that seems possible. We know that before it reaches some sort of agreement, there're various procedures to follow and it is also possible that there will be debates and arguments to follow.
Although the change of currency system is possible, I don't think the US will follow through with this, not even if their country falls to shitter. Besides the cost, and the impending and much worse financial collapse that this will bring, they'd also be taking everyone with them. At least this is the case if ever US is to change financial systems.
For other countries, it's very much possible as what the other peeps have told, even if no public consensus was made, as long as you have enough reserves to back the value of your currency, much like what happened in China and their Yuan, you should be good to go.