Assuming it's about Character Development and not just switching hat: the one that hit his Berserk Button was a very rare official Pokémon League Expo hat that he won by sending about a million postcards (or so he claimed).
Ash's other berserk button is if anyone harms/kidnaps/insults Pikachu. Cases in point: Pikachu got kidnapped by Hunter J and Ash gets very, VERY pissed off, eventually starting with jumping onto a moving car, then sneaking aboard J's ship with Jessie and James (who are also going to rescue Meowth), he gets in a fight with Lucario when it insults Pikachu in Lucario and the Mystery of Mew, he rescues Pikachu and takes a Poison Tail from Seviper in the back to protect Pikachu, he attacks a machine to break Pikachu out in Mewtwo Strikes Back and does a similar thing during the third Sinnoh episode. It goes for his other Pokémon as well.
He also hates it when someone insults his friends. When Alpha Bitch Ursula in the Sinnoh series nastily taunts Dawn and calls her pathetic, for example, Ash is VERY quick to backsass Ursula and defend Dawn.
If someone deliberately harms Ash, then EVERY every one of his Pokémon will pounce on you, hard. The Team Rocket trio, being the Chew Toy group that they are, managed to do such a thing just at the right moment that Charizard had finally learned to respect (and care for) his trainer (Charizard Chills, episode 105). VERY OUCH for them.
And just as ouch when they did it in front of his then-depowered Sceptile. The mon was so pissed off at them almost throwing Ash off their globe that it regained all of its power out of sheer hot-bloodness, rescued Ash and Pikachu and blasted TR like whoa.
Another Berserk Button that doesn't come up often: Ash is a Momma's Boy. Very, very much. Do not speak ill of his mom. Do not gawk at her, either. And do not try to harm and/or abduct her. Ash was ready to punch out a legendary Pokémon over this.
Threatening or hurting his other human companions will also not turn out well for whoever does it.
Ash doesn't take kindly to other trainers mistreating Pokémon either. Case in point: in the early episode with A.J., Ash flew off the handle when he thought A.J. was beating up Sandshrew (in truth, he actually cared for his Pokémon).
Fall asleep while Jigglypuff's performing and it will get angry. Steal its marker/microphone or otherwise deny it the chance to perform and it will get REALLY angry.
Brock does NOT take kindly to trainers abandoning their Pokémon. Neither do Ash or Misty.
Jessie and James will explode if someone messes up their motto.
James also is extremely protective of his bottle cap collection. If you damage it or steal it, or even mention doing such, he will flip out.
However, as seen in Mewtwo Strikes Back, they don't mind if even one of the "twerps" (in this case, Ash) stops their motto before it begins.
Never call Jessie "a hag" or "old". It's a good way to ensure a death wish.
Heaven help you if you mess up or insult Jessie's hair as well. This is one of the many things that can easily set her off. Poor Seviper... she defeated (and subsequently caught) him bare-handed after he accidentally tore it off!
Ash's Infernape will give you a good fight. But seriously, don't taunt him for being weak if he's losing. Especially if you're Paul. You're likely to trigger Blaze, and have a Crazed-Fire-Saiyan-Monkey on your hands.
One of the Pichu brothers, in Camp Pikachu, gets to have its own Berserk Button, complete with an anger cloud, when Sudowoodo steals the grapes.
Also, in one episode, Phione realizes that Buneary likes Pikachu. Needless to say, it does NOT look pleasant. Phione has fire in its eyes, and yells at Pikachu.
Cilan gets outraged when he sees Skyla's method of 'air battles', which are just battles in her head instead of real battles.
Also, do not try to disrupt Cilan's cooking in any way.
Dawn cannot stand it when someone she knows doesn't remember her. One time Paul said he didn't remember her, which he probably said to piss her off, made Dawn ready to attack him. This could also double as a Crowning Moment of Funny.
Just try insulting Brock's or Misty's gym and see what happens.
Misty won't stand for water type Pokémon in general getting insulted.
Likewise insulting Norman or Petalburg Gym in front of May or Max.
Meowth is not a pathetic Meowth and don't tell him so. The Meowth in boots in one Hoenn ep did so, and he was not pleased.
Also, Meowth does not like his charm being lost/misplaced. It wasn't really lost in the ep where it disappeared, but he was ballistic the whole ep wanting it back. I wouldn't suggest stealing it.
He doesn't like seeing the other Meowth get frozen by the Pokémon League test supervisor's Vaporeon in "The Ultimate Test", though he is perfectly willing to use Fury Swipes on another Meowth himself later on (made worse in the Hoenn League because the other Meowth called him 'pathetic' as mentioned above).
Meowth also refuses to back down to Persians. Meowth-in-boots also has this trait, Up to Eleven.
Do not call Giovanni in the middle of the night/early morning. It won't turn out well.
Misty does not like her sisters calling her a runt.
There is also a Marill that appeared in the Pikachu Specials. But watch out! It's got a Sing attack... with one of Jigglypuff's berserk buttons. If anyone falls asleep, it gets mad and inflates itself. Just like Jigglypuff! And then, Marill hits the asleep victims with its tail.
Do not hurt/bother Togepi or you will face its friends' wrath.
Don't even think about selling Pokémon in front of Professor Oak.
If you try to claim that Pokemon battles aren't art around Siebold, he absolutely loses it.
Silver and Blue were raised as part of the Mask of Ice's recruitment and training campaign, and the two of them have developed a bond of their own (whether it be one of siblings or possibly more doesn't really matter - that they're precious to one another is). Green's "noisy woman" comments about Blue are responded to with a glare from Silver as a reminder that he is on notice; if you harass or bring harm to her, he will turn you inside out.
To say nothing of the beating he gave to Gold for touching her...
For that matter, Red was furious when finding out that Team Rocket were the ones who ruined Blue's long-awaited reunion with her parents, to the point of charging recklessly towards the Beast Trio even when they had the upper hand on him.
Don't kidnap or harm Green's grandpa or he will find you and use Porygon as a virus to destroy your circuitry, along with any of your predicted, inevitable outcomes.
When Team Rocket starts fighting in the Viridian Forest, Yellow gets just about ready to kick their asses; her usually weak Pokémon's levels jump up about fifty levels in response to her rage.
Lance triggers a similar response during his attempt at genocide. The moral of this little story is that Yellow is slower to anger than just about everyone else on the planet, but once she's pissed, you call your Pokémon back, you turn one-hundred-and-eighty degrees, and you never stop running while there is breath in your body.
Pokémon Diamond and Pearl Adventure
Hareta has a hard time understanding the concept of evil - one of his buddies is an ex-Galactic, he lets several wrongdoers slide (though he does stop them from time to time, it's just what he does), and he even has to explain to Cyrus that he doesn't hate him for all he's done. But if you sell Pokémon in front of him...
The long lasting feud between Zangoose and Seviper is so intense that the very sight of one another makes them feel nothing but rage, leading to an immediate battle. It's so bad that in horde battles as featured in Pokémon X and Y, if 4 Zangoose and a single Seviper are in the horde, the Zangoose would rather gang up to take down the Seviper before attacking your Pokémon. This also applies when it's 4 Seviper and one Zangoose.