I dont think people will swap to FIAT money, they would just swap bitcoin straight to goods.   This is what happens with gold bars when buying property in Vietnam because they dont want to risk losing value during the exchange, its easier just to keep hold of the gold which will adjust in price and literally its done as a trade or exchange of shiny bar for the land with no paper money.

Theres the famous tale of the family in the 1920's selling all their goods and house in Germany post world war one and their purchase for all these things in a distressed market was ferry tickets and travel out of the country to a more stable environment to work.   So they sold the goods and received their cash which was a currency with hyper inflation and by later that day they got to the Ferry to buy the tickets, the price had already moved too far and their money was now not enough for all the family to travel.  
It becomes too dangerous to hold money eventually, right now for western countries the danger period is maybe ten years.  You cant save for retirement with cash because its going to fail to buy your food when you cant work from old age.
  As the currency standard falls apart the one decade becomes five years, then a couple, a year then its just months before you lose too much value to ever hold on.  It accelerates, Im sure you wont be swapping to FIAT when it gets that bad you will hold onto assets and swap over and its then bitcoin is useful for being immediate and accurate