I glanced at “I”s for one state-—can’t remember which-—I think NM. They had declined, so no reason to think that Ds are just becoming “I”s. But you’re welcome to go to the NC SecState site & look at November 2016 “I” and compare with June 2017.
NC’s unaffiliated number for 7-1-2017 is about 1/3 of their total registration.