The humble pawn, which upon reaching the last rank on the chessboard gets promoted, almost always into a Queen, the most powerful piece in the game
The Queen herself. Initially the piece was known as the vizier and moved very similarly to a pawn (i.e. it was much weaker than a king, which can at least cover all eight squares around it). Then, during the 15th or 16th century, it was given it's current movement abilities and, well, there's a reason the pawns tend to get promoted to queens.
This upgrade (orthogonal and diagonal travel without limit where unobstructed) became tellingly known as "madness chess" ("That much power wielded by a woman? Madness!") and the Italians called the piece rabioso.
If it comes to that, the ancient bishop was even weaker, moving diagonally two squares exactly, which restricted it so that 56 of the 64 squares were permanently off-limits.
Shi Woon, protagonist of The Breaker, takes several huge levels in badass over the course of the story. He starts out as a loser who gets beaten up by bullies daily. After he eats a strange pill and trains under one of the greatest martial artists in the world, he becomes strong enough to break a cement column with his bare hands and take on other more experienced martial artists. He also learns to stand up for himself and becomes The Determinator, which impresses even the antagonists he cannot defeat.
Elefseus in Sound Horizon's Moira. After his sister is killed, he turns into Amethystos, prince of taking down fate.
Paul McCartney's Live and Let Die says that "you used to have a heart like an open book and say live and let live, but if this world in which we're living makes you cry...say live and LET DIE!!!" Break the Cutie into Badass.
Katy Perry's Roar is about a girl who starts as a Shallow Love Interest, but after breaking up with her boyfriend goes on to become a strong independent woman. The video goes one step further as Katy plays a city girl whose plane crashes in the jungle, and when her Indiana-expy boyfriend is eaten, she levels up into a Jungle Princess.
Dick Tracy had Pat Patton, who was originally a Bumbling Sidekick for Tracy in the first few years. However, before the 1930s ended, Pat grew to be a hardheaded and reasonably efficient police detective who can find the important clues and save the day when called upon.
In Metroid Prime, the Triclops was a minor annoyance that just latched onto the Morph Ball and spit it out for minor damage. But in Metroid Prime Pinball, it's a major problem due to its tendency to release Samus at an unfortunate angle.
Happens all the time in sports, especially when a team goes "worst to first". Very common in Baseball.
Happened to TWO teams in 1991, when then-perennial losers the Minnesota Twins and Atlanta Braves, met in the World Series. The Twins won it all last year, but the Braves would go on to be a force for the next 15 seasons, ultimately winning it all in 1995.
This is particularly important for Braves fans; it signalled the end of the "rotten years", a period from 1970 through 1990 where they lost a lot more often than not, only making the playoffs two years out of 21.
The Tampa Bay Devil Rays were...slow out of the gate, having losing seasons their first ten seasons, finishing DFL in the AL East nine times out of ten (they finished 4th out of 5 in 2004). Then in 2008, they dropped "Devil" from their name. They immediately won the AL East (keeping the Yankees out of the playoffs), then beat the Red Sox in the ALCS to make the World Series. They've had winning seasons ever since, making the playoffs in 2010, 2011 and 2013.
As a player example, Patty Mills of the San Antonio Spurs basketball team. During the 2012-2013 postseason, he was widely known as the chubby guy sitting on the bench, cheering his teammates on by enthusiastically waving a towel whenever they scored a basket. After his team lost the championship, he was determined to prove that he deserved time on the court, so he went on a strict diet, lost weight, and trained rigorously. Flash forward to the 2013-2014 season, and he's been absolutely killing it as the backup point guard, helping the Spurs to maintain their top-seed record by stepping in for the injured Tony Parker and going off for 30 points or more in his best games — a number that would be considered respectable for superstars like Kevin Durant and Lebron James.
As with the cartoon series, so too with the toy line it's based on. When Transformers makes a new toy of an old character, the advances in manufacturing technology usually mean the toy itself is better. It is also often larger. Recent examples include Seaspray, once the Aquaman of the Autobots, who has a toy that is apparently both his G1 character and a character for the new line, going from an equivalent of Legends to Voyager (three size steps up; the original is three inches tall while the new one is over a foot.) This can even happen within a single line. In the Revenge Of The Fallen toy line, Bludgeon was originally a repaint of Wreckage, a decent mold from the first movie's line, but got his own unique mold which was a full size class larger, more in tune with his fan-favorite G1 self, and dripping with flavor.
Mata Nui from BIONICLE seems to go down a few levels of badass at first, after which he explains that he never actually was that badass, just very powerful. Then he throws a Humongous Mecha under a moon/fragment of the planet he's standing on, crushing the Big Bad.