Earlier quoted context omitted.
Fiat is slightly different in that the government can guarantee a certain level of demand for it by requiring taxes to be paid in it. If you you don't like jail and have a tax liability in US, you can't think dollars are worthless.
> Fiat is slightly different in that the government can guarantee a certain level of demand for it by requiring taxes to be paid in it Until they switch currencies. For instance Brazil did this > The cruzeiro (Portuguese pronunciation: [kɾuˈzejɾu]) was the currency of Brazil from 1942 to 1986 (two distinct currencies) and again between 1990 and 1993. In August 1993 it was replaced by the short-lived Cruzeiro Real, it…
But people still need it to pay taxes and the government will only pay for services with it.
In most developed nations, the government is responsible for 20%+ of the total GDP, so that's an inbuilt "floor" to the value of its currency.
So yes, the government can change the currency and the notes, but they control the supply and demand of whatever they deem to be the currency.