My nonskid had been primed and painted with Interlux Brightside and, maybe, once before I bought the boat in '91, with something else. The results were good but the paint builds up in the valleys between the nonskid grid, and over time the peaks of the grid wear down a bit. As a result the depth of the nonskid starts to be reduced. This may or may not affect traction when dry, but it almost certainly reduces the speed that rain or splashed water gets off the surface.
Once I start cutting into the deck for core repair, the jig is up for the nonskid.
There is nothing magic holding this old paint onto the deck. So to give new KiwiGrip a relatively uniform surface and the best chance of long-term adhesion, before I tape off and apply it, I take the old nonskid down with a 40 grit disc on a grinder until I can't detect the pattern with my fingers. This is exceedingly easy, quick, nonprecision work, but it does throw a lot of dust, so I recommend wrapping not one but TWO brightly-colored bandannas around the mouth and nose while doing it.
This treatment often also reveals a surprising number of former deck holes from long-abandoned hardware that had been inadequately filled. Better to remedy these situations now rather than deal with the aftermath!