To my knowledge, from the development of bitcoin usage until now seems there has been no plan/news on bitcoin will be possible to shifting fiat currency, and if it will be planed certainly the governments will more consider it again and it will not be easily to decide it. But I think logically, I'm not believe that it is very likely to happened.. I think that until the future bitcoin will remain as a digital currency that can be said as an alternative currency from a fiat currency in general, and will always coexist throughout history.