If you modify the vehicle in a way which means that it ends up in a tax group with a higher one-off registration tax, you will have to pay a supplement. This supplement corresponds to the difference between the tax on the original vehicle and that on the vehicle after it has been modified.
January 1st 2017 the effect component was fased out for all tax groups with the exception of tax group F (motor cycles) and G (snowmobile). An increase in effect will not be evident in the one-off registretion tax, but might influence the vehicle's emisson.
It can be a good idea to check with the Public Roads Administration if you are unsure.
The additional tax will be reduced depending on how long the vehicle has been registered in Norway.