The Breaker started out as a fairly lighthearted action manhwa with plenty of comedy thrown in, but gradually got more serious as a major character's Dark and Troubled Past (and its present consequences) comes to light. By Volume 10 the humor is entirely gone and another major character has been killed off.
Let George Do It initially started out as a comedy about a soldier back from the war going into business as a professional odd-jobs man, doing things too silly or embarrassing for others to do, including occasional work as a private detective. He had a lovely young woman to assist him, with a gee-whiz little brother to get into light-hearted trouble. Over the course of several episodes, however, changes like the sudden disappearance of the kid brother and the music going from full orchestra to organ-only darkened the tone of the show to the hard boiled detective series that the show is known for being now.
Everyone goes through this. When you're a kid, you've got no responsibilities and are, with some exceptions, free from the drama of the outside world - then comes the windfall of relationships, bills, jobs, money management, jury duty, and eventually death.
The introduction and the first arc of Yu Gi Oh East Academy are both fairly standard Yu-Gi-Oh! fare, with rivalries and friendships developing and generally a fairly light-hearted roleplay. Then the second arc rolls around...
Destroy The Godmodder: Began out as just a goofy game that was for fun and didn't have much of a coherent storyline. Act 2 of the second game hits and suddenly, *collective gasp* there's an actual plot. Complete with downers, sudden twists and the like.
Admittedly it almost strayed here near the end of the first game, but it quickly went right back into light-hearted and silly.
And it wasn't sudden, it happened gradually as several players started roleplaying and coming up with actual ideas. The Split Personality disorder craze didn't help any either.
William Shakespeare wrote Measure for Measure while composing his greatest tragedies. Described as his "farewell to comedy", it ended in weddings (as all his comedies did) but had very little to laugh about. It was also the last one he wrote, except for The Tempest and Merry Wives of Windsor.
Next To Normal is all fun and jokes for most of the first act, until Gabe is revealed to be dead. It only deteriorates more in the second act.
Most of the first act of Wicked is a light-hearted story about a green girl trying to fit into school and becoming friends with her popular, ditzy roommate while also falling in love with the class clown. By the end of the Act, culminating in "Defying Gravity", Elphaba discovers the truth behind the Wizard and vows to right his wrongs, getting her labeled as public enemy number one and having her best friend choose fame and power over the side of good and truth. That's just Act 1; it gets much worse in the second act.
Into the Woods is all wishes and dreams coming true in the first act but then Act Two begins and the giant shows up.