This could not happen, not every country is modernized enough to replace their currency for a virtual currency, this would not happen anytime soon, I do not think it will even ever happen.
But this virtual currency has the potential to turn users who are using their bank cards into using their bitcoin wallets. Yes the ration of bitcoin users will beat the traditional fiat users in near future for sure. The raising user bases and bitcoin adopting new merchants are some visible proof to that.
Replacement in the sense It will replace International transaction and this bitcoin will help more to common people who is not able pay to many taxes and other hidden charges. Instead of saving separate country banking system which have link International reserve banking system will get damage because of bitcoin transaction method. Thats why government is not allowing bitcoin.