Darker and Edgier/Web Comics

Sailor Moon CS compared to the anime it's primarily based on
The superhero parody webcomic Emokid and Chemokid was initially dark comedy, but eventually became a suspensefully story with deep backgrounds and character development while still maintaining the odd sense of humour that only the writer of Head Trip can provide.
Spoofed by the Stick Figure Comic Stickman and Cube in this strip. The cartoonist announces that he is making the comic "Darker And Edgier", gives the characters new Wangst-ridden backstories and sums it up by saying that "basically everyone's just going to shoot each other and swear a lot." At which point Stickman says "Oh, HELL no." The characters later get back at the cartoonist for this. With extreme prejudice.
In a May 2008 Dominic Deegan strip, the title characters learns, to his horror, that his favorite comic is about to become Darker And Edgier... by retconning the hero into a demon. It's like making Superman Darker And Edgier by giving him a German heritage, a small mustache, and a great personal hatred of Jews.
- It was foreshadowed at the end of the Storm of Souls arc. One of the cosmic consequences of breaking the storm was that many of the souls normally bound for Heaven or Hell ended up in the wrong place.
Butterfly, an Affectionate Parody of Batman, spoofs the phenomenon here. The specific point of reference is Christopher Nolan's latest Batman films.
Occurred in A Modest Destiny, around the third story arc. Up to that point it was a lighthearted and humorous fantasy spoof. Then came "The War of Fate" arc, which was much more darker and serious, such as the female lead considering getting an abortion.
The original material that Exterminatus Now is based on was 'The Grim Darkness', a project to make the Archie Sonic the Hedgehog comic universe GRIMDARK by blending it with Warhammer 40,000. According to Word of God: "...in reflection, one of the worst ideas I have ever had." The actual webcomic makes fun of it.
Ctrl+Alt+Del. Let's not forget the shift from "two game nerds on a couch" to "personal responsibility and miscarriage tragedy".
Wapsi Square has certainly gotten much darker and edgier since it shifted from a slice-of-life comedy, to a supernatural drama; dragging in themes of suicide, human sacrifice, manifesting personal demons, and a looming quasi-apocalypse.
Breakfast Of The Gods is a Darker and Edgier and Bloodier and Gorier use of breakfast cereal mascots, of all things. Tightly-written and exquisitely drawn, it's D&E&B&G at its finest. Frankenberry is a sadistic thug, Sonny the Cuckoo is batshit insane, The Trix Rabbit is a private investigator, Toucan Sam runs a bar and acts a lot like Rick. It's now completed.
Shortpacked! mocks this tendency here and again in reference to G.I. Joe: Resolute.
Parodied in Xkcd, here. Harriet The Spy??
The concept is mocked in this Fey Winds page.
Sonichu started out with Chris-Chan's poor attempt at making a crossover between Sonic the Hedgehog and Pokémon. In the last three issues, we've seen his electric hedgehogs have sex in graphic detail, Chris murdering trolls by the hundreds, and Chris torturing people in the most inhumanly brutal ways possible for... making fun of him online.
Hijinks Ensue's (dark) MacGyver parody can be found here.
DMFA started out as a very light-hearted comic that basically followed the everyday antics of Dan, Mab, and their friends and family. Later on, things like Dan discovering that he is an incubus, Mab being involved in...something that she claims will kill at least two of her friends, a flashback revealing that on Dan's first adventure one of his friends was brutally killed by Dark Pegasus, and all of Abel's Story made it clear that DMFA is definitely a more mature comic than when it started. Though this to be expected, as the comic was started when the author was in high school, and so it's probably matured along with her.
Formerly Lawful Stupid Cale, from Looking for Group has certainly come a long way since the comic started. Due to the influence of undead warlock Richard, cynical companions Pella, Benny, and Krunch Bloodrage, and the fact that his new Kingdom was Powered by a Forsaken Child, Cale has become far more flexible with regards to his morals, and even smiles when he sees an old foe intimidated by Richard's undead townsfolk.
Homestuck has several points where it gets darker and grimmer: playing a The Sims Meets EarthBound game causes The End of the World as We Know It, when we learn that Anyone Can Die and winning the game won't save them, when Eldritch Abominations are revealed to be real, when we meet the Trolls and their Crapsack World ends, and most recently when Rose has started to Screw Destiny with a passion, aided by the aforementioned abominations. Fortunately it never gets too dark:

tentacleTherapist (Rose):I'm not actually trying to caricaturize a grim sorcerer.
TT: There's still a perfectly intact piece of my mind which realizes how ridiculous it is to be flying across rainbow oceans with a couple of magic wands and a salamander in a little cowl.
[a bit later]
ectoBiologist (John): maybe later, i will drop by your planet again and rescue you, thus breaking the spooky spell put on you by your nefarious, shadowy masters.
EB: that way you will stop being so grimdark and ominous, and basically completely off the deep end in every way, as is now painfully obvious to anyone with a brain.
TT: Swoon!

- Oh, it gets dark all right.
- And of course, there's when Rose actually goes grimdark.
  - Well, dying and resurrecting as a God Tier put an end to that. And with the end of Act 5, it seems like things are going to get a bit less dark.
  - Ah, no. And that is the Author Avatar getting killed there.
Sam and Fuzzy parodies the trope here. And then again here. And then manage to further parody the second parody here. Sam Logan does not seem to like this trope.
Full Frontal Nerdity shows one possible way of how this works.