For anyone not too familiar with comics, or DC comics, they have a lot of "legacy characters". There may be a superhero with one name, who dies or just disappears or the comic gets canceled, and they come up with another character who takes up the torch--they get the same powers and take up the same name. So there have been several Flashes, several Green Lanterns, a couple Blue Beetles, several Starmen, some Firestorms, etc. A lot of the original characters were white (they were created mostly several decades ago, so ethnic diversity wasn't really considered), but some of the legacy characters have become more diverse. So we had a black Firestorm, a Hispanic Blue Beetle, etc.
Mr. Sims's article is about the tendency of comics writers to basically kick the new character by the wayside and go back to the "original" character--which results in an unintentional whitewashing, because the newer character gets sidelined, or replaced entirely by an older, established white figure.
But now, the idea of a legacy character is being totally subverted. They're not roles that are passed down anymore, they're roles that are passed back up.
And much of the time -- not always, but enough that it's more than notable -- they're being passed back from a non-white character to an Aryan ideal. Jason Rusch is still part of Firestorm, but it's back to being Ronnie Raymond's Caucasian body. Kimiyo Hoshi is still Dr. Light, but that name's been permanently soured by "Identity Crisis" and the fact that James Robinson had the original Dr. Light threaten to rape her children on the Justice League Satellite. Even the regressions of ostensibly white characters often have racially charged consequences: Wally West's interracial marriage to Linda Park has been sidelined in favor of on-the-go suburbanites Barry Allen and Iris West, and Kyle Rayner (who was created as an Irish-American but later "revealed" to be the son of a Mexican-American CIA agent) has suffered the strange fate-worse-than-death of a fictional character who gets demoted from a starring role to a supporting one. He's still a Green Lantern, but he's not the Green Lantern.
It's a well-written article, and amazingly enough, almost all of the comments (as of the time I'm writing this) are supportive.
[Edit] A picture of a recent example of this (Firestorm reverting to the dead white guy instead of letting the alive black guy continue in the role) as depicted in cartoon format, by the same author:
You can see the whole thing here.