The government have never changed the banding of a vehicle retrospectively. The changes have always been for vehicles registered at some point in the future.
Thats why we have separate tax regimes for cars registered before 2001, between 2001 and 2017 and 2017 onwards.
I would therefore expect a petrol & diesel cali registered today to always be charged exactly the same road tax.
It doesn't stop them putting the rates up though.