I realized I don't think I asked the question as I wanted. In this scenario the country doesn't switch to the coin, the currency is just BTC, built upon BTC, and will continue to rely only on BTC, not some foreign currency like USD or Euro. What I'm thinking is just a new country. Like the idea of Esperanto land.
Asgardia .. it's a nation that formed off the earth could be the first doing that 
. Any country in this earth wouldn't be able to fully using bitcoin as the main currency ... as the main financial system, it would takes really long time to make it happened. I heard also Venezuela trying to do this as their financial crisis getting worse, now I didn't hear much anymore about this. Anybody have a clue? Or it's simply totally failed ..