how will people adapt change from FIAT CURRENCY to DIGITAL CURRENCY like BITCOIN?

Start with online banking, introduce backend as blockchain related currency, have the app auto convert the value.
That might confuse people since their balance is changing so probably would need a cash side and a cryptocurrency side with a note on the screen that the prices are changing over time.
Done and implemented.
I think it's the opposite way around. Digital to fiat is the better one. Have you already tried that one that you mentioned above? It also confuses me too. I guess it's better if cashing out money from crypto to make it fiat and that's the time you start your bank account,not the online banking. I think it's a lot worth it so that you can ask more information directly to the bank or if you have complains. As you mentioned,the balance is changing and it needs cash that's why I prefer to fiat money first so it maintains the balance and with less worries.