RBI can surely ban Bitcoin any time they want. But they will give you a timeline to come out of your accumulated Bitcoins. However, you will have to file your taxes in a very diligent way. So you will get a time to sell off your holding before the ban comes into effect.
But honestly, I don't see it happening. A blanket ban is highly unlikely to happen in India now.
Fact that RBI already got one backlash after placing a blanket ban, they will likely not burn their own faces again with a similar fiasco.
In am sure the people who run the show there have understood that bitcoin by design, cannot be banned by a country's central bank.