Bristol Motor Speedway announced that they are grinding down the upper groove of the track to decrease the progressive banking. My (to-scale!) sketch of what I think they are doing from the press conference and the tweets (thank you so much Nate Ryan!) is below. The 2007 re-do introduced progressive banking, with essentially three lanes. The lowest lane had 24 degree banking and the upper lane (nearest the wall) had 30-degree banking. I’m guessing that the middle lane was 27 degrees – haven’t been able to verify this, but it’s probably not far off. (My values are from the Bristol Motor Speedway website.)
The change (again, I’m inferring this from the press conference and tweets) is to decrease the banking on the upper lane. Assuming they make the banking the same as the middle lane (their graphics seem to suggest this), the new surface would look something like the line in red.
Why does this make a difference? Briefly, banking helps a car turn. A car driving into the page needs a force left to make a left turn. The banking provides additional force, which adds to the force provided by the tires. The force scales like the speed squared – the more turning force, the faster you can go. All other things equal, a car goes faster on higher banking.
If a track is uniformly banked, going around the outside of the track gives you no advantage: you have the same turning force, but the outside of the track forces you to drive a further distance.
By putting the progressive banking at the edge of the track, you increase the distance the car has to travel: the advantage in speed you get from the additional banking is (somewhat) offset by the longer distance. How effective this is at creating more grooves depends on the relative advantage of distance vs. speed.
There are other factors, such as how much rubber is laid down in each groove, and setups that may make the car handle better in one part of the track than the other, that have to be considered.