Regardless of which 'side'' people fall on in the discussion of the change of pace in Vinland Saga season 2, everyone can agree the show massively switches things up between its first and second seasons (intentionally so).
Granted, Vinland Saga was always planned to be a story about a Viking fighting against the violent ways of that time period, but it still took viewers a while to get used to the new pacing and tone of Ketil's Farm in season 2. So, with that in mind, let's go over a couple of other anime with big sudden shifts like Vinland Saga.
8 Berserk
Let's start things off with probably the premier example of a big sudden switch-up, Kentaro Miura's masterpiece, Berserk. There are many who describe the Golden Age arc of Berserk as the 'introduction' or 'tutorial' arc. And, while that is a relatively accurate description, people need to remember that this arc is still 11 Volumes long in the manga source material.
So, while manga is easily the recommended way to experience this story, if viewers see either the 1997 anime adaptation or the 2016 one, they'll be witness to a few massive tonal changes throughout the course of Guts' journey.
7 Hunter X Hunter
Next up is Hunter x Hunter, arguably one of the most iconic Shonen series of all time and one that has such an iconic art style to it since it's so cartoony but the series overall is incredibly dark.
Fans don't get a full picture of how dark this story is going to be until after the Yorknew City arc, though there are more examples of major shifts in this series than just the big one during the Chimera Ant arc. The Greed Island arc, for example, changes the actual genre of the series from a shonen battle anime to a card battler survival game anime.
6 Season 2 Of Megalo Box
All of the recommendations on here are anime that go through big shifts in their story or setting at some point, but it's usually a shift from low-intensity to high-intensity, AKA a peaceful time to a turbulent time. But, this is the opposite of how Vinland Saga season 2 was. However, Megalo Box season 2 is by far the most similar example of Vinland Saga season 2, as it trades the high-octane boxing action of season 1 for a much more realistic but sad story in season 2.
Season 1 of Megalo Box is this high-intensity climb of Junk Dog up the pro-Megaloboxing ladder all the way to Megaloboxing Champion, all while a single punch could likely kill him due to his moniker of 'Gearless Joe'. The second season, however, is a much more introspective journey of Junk Dog, who now goes by 'Nomad', as he's dealing with a painkiller addiction and what it really means to be 'champion'.
5 Mushoku Tensei: Jobless Reincarnation
Jobless Reincarnation is one of those anime that is incredible for those who commit to watching it, but very hard to recommend to people relatively new to the anime medium. The show has a lot of 'mature' themes and a whole lot of fanservice, but at its core, this is by far one of the most beloved isekai series in existence and is comparable to other isekai-esque icons such as Solo Leveling.
And, while it may seem more like a comedy than an adventure anime at times when it comes to any of the episodes labeled 'Turning Point', fans can buckle up for some real emotion and dramatic twists. The first half of Mushoku Tensei, for example, is almost an entirely different show than the second half of that same season. And, season 2 (based on what happens in the manga source material) will also be switching things up quite a bit.
4 Ranking Of Kings
Ranking of Kings is the show on here that most loosely fits the criteria of switching things up but for good reason. While this show doesn't really have a 'sudden gigantic shift' in atmosphere or tone at a specific turning point, it does have these smaller moments of radical and unpredictable change in almost every episode that separates it from other similar series.
So, fans aren't really dealing with a sudden massive change in its storytelling, but in exchange, it's almost impossible for first-time viewers to guess what's going to happen next. All that said, there are a few moments in the first season of the series where the plot takes some sharp turns, such as the end of episode 4, episode 11 onward, or the last few episodes of season 1.
3 Jujutsu Kaisen
At the time of writing this, the second season of Jujutsu Kaisen has only a few episodes out, but based on how closely the first season followed the source material, fans of the manga know that there's going to be a pretty massive shift near the end of this second season.
Not to get too much into spoilers, but basically, some events that will happen likely near the end of season 2 will massively affect the story going forward and will blow viewers' minds, regardless of them knowing something might happen long in advance.
2 Cautious Hero: The Hero Is Overpowered But Overly Cautious
Overly Cautious Hero is probably the softest switch-up on here, but it absolutely still deserves a spot. This series, at first, seems very one-dimensional in its comedy. The Hero named 'Seiya' is overly cautious to an impossible degree, Ristarte (the goddess who summoned him) gets frustrated with the Hero, and the situation in this S-Ranked world of Gaeabrande somehow surprisingly proves that Seiya's caution was warranted.
However, the story really changes pretty dramatically in just a few episodes in a way where Seiya's impossibly paranoid ways are less frowned upon, and new characters are introduced that play parts in events that are much darker than that first episode may make things seem. Also, this series has some real moments of suddenly becoming a lot more 'adult' than it may have seemed at a glance.
1 Chainsaw Man
And finally, let's talk about one of the more beloved anime adaptations in recent memory, Chainsaw Man. Chainsaw Man is a roller coaster ride all the way through, and fans of the manga can back this up. However, even in the single season of the anime (and most other anime like it) that has come out so far, the series has some pretty massive turning points both in terms of drama and tone.
And, these turning points only become more and more prevalent as the story continues, especially for the last 'arc' of Chainsaw Man part 1. Just to name a few big moments of radical change without spoiling too much, there's the introduction of a Primal Devil, the reveal of a character's long-con, and a character's sudden realization of their actual wants and needs.