So, two days ago I finished Jak II and it honestly shocked me how different the game is from the first one. They went from a perfectly innocent child-friendly platform game in an almost fairytale setting to a still cartoonish but pretty dark GTA III clone with quite a lot of violence, guns, sandbox gameplay with vehicles in a dystopian city... the list goes on. I was frankly very disappointed and even a bit disgusted by the direction they went in.

I'm wondering what other examples there are where the developers radically changed almost everything in a sequel. Whether it's a radical change in tone and style or basically a switch to another genre. I don't quite mean natural changes like e.g. a transition from 2D to 3D in the late 90s but completely unexpected radical changes to the whole thing, especially ones that big fans of the previous game(s) certainly didn't expect and probably didn't want. Spin-offs like Halo Wars or The Bureau: XCOM Declassified, which do not affect the main series, don't count.

Another example I can think of would be Ecstatica II. The first game was basically a survival horror game with some clumsy average Joe protagonist in a kinda authentic medieval town. The second one was suddenly some fantasy slasher game where the hero has been retconned into a prince and slices and dices his way through hordes of continuously respawning enemies.

The ones that stick out for me are Lords of The Realm 3, and Command and Conquer 4. Both completely threw away the old mechanic and "tried something new". In both cases they really disappointed fans who'd come to expect that with the name comes some similar kind of game.

Super Mario Bros. 2 (USA); note that the game called Super Mario Bros. 2 in Japan is very similar to the original
Zelda 2 (side-scroller, also has XP but no currency)
Castlevania 2 (could maybe be classified as a metroidvania, one could argue it's like later Wonder Boy)
Fire Emblem Gaiden (has non-combat exploration, no durability, spells use HP)
Final Fantasy 2 (ditched classes and XP in favor of a classless system where actions determine stat growth)
Ys III: Wanderers of Ys (a 2D sidescroller, unlike the rest of the series; ironically, its 3D "remake" actually has more platforming than the source material)
Super Mario World 2: Yoshi's Island (does not play like a Mario game at all; instead you control Yoshi, don't get clasic Mario power ups, but you do get to swallow enemies and turn them inti eggs that you throw)
Castlevania: Symphony of the Night (a metroidvania, unlike previous games in the series)
Super Mario 64 (it's not just the move to 3D here; the health system works different, the old power ups (lkke the fire flower) are gone, and Mario can now punch)

One could also put Final Fantasy 11 in this same category simply because it's numbered like a title in the main series; if it had been titled differently (Final Fantasy Online is the title I would use), then it (and its sequel (FF14) which would be called Final Fantasy Online 2) would count as spin-offs.

The jump from Prince of Persia: Sands of Time to Warrior Within was pretty jarring.
They spiced up the combat, but at the expense of the fairytale arabian nights tone Sands of Time had- that was replaced by a grimdark brooding hero, screamy metal music (not bad quality music necessarily, just giving a very different tone from the original) and an attempt to be more "adult" (but in a juvenile way).

Carmageddon went from 2, a game with fun exploration focus and wacky misadventures in wide open maps to TDR with time limits that really narrowed down the exploration in favour of shiny slickness.

Then there are the transitions from adventure game franchises to action games of the 90s, but I think that might almost come under the "natural" you were talking about :D- Indiana Jones went from Fate of Atlantis to Infernal Machine. King's Quest went from cartoony almost casualness (also a huge jump from the previous game in the series) to bland actiony game with RPG elements.

Mentioning Indiana Jones reminded me of how this has been done many times with movies where they bought the rights, used the title and character names, and did their own thing with it that hardly resembled the original (which might have been a movie, TV show, book, song, or something else). Offhand I am thinking of The Hobbit, Sherlock Holmes, and Mission Impossible.

Edit: Oh, and Excalibur. Wasn't bad, just not similar to what the title suggested.

Not sure what you mean. John Boorman's ? It's the most accurate representation of the arthurian cycle that I know of. Except maybe Eric Rohmer's Perceval, but haven't seen it yet.

And I'm speaking as an avid reader of XIth century litterature.

Given the username of the topic creator, how come nobody has mentioned Fallout 3? From what I understand, that game is completely different from previous games in the series, to the point where one could reasonably not consider it to be the same genre.

It had occurred to me as well, but other than the move from 2D to 3D (which the OP specifically stated isn't what he considers a radical change), the game actually is still reasonably similar to the first two, IMO. You still have skills and perks, you still get different factions in the wastelands, etc.

I know I'll probably get a crapton of flak for saying that, but that's how I see it.

Yeah, man, so edgy! *facepalm* It did come across as them trying too hard. I still enjoyed the gameplay though. Sands of Time is still the best of the trilogy.

There's other changes, like the change from turn based combat to real time combat, that make it completely different.

In particular, under what I would consider a reasonable definition of RPG, Fallout 2 would qualify, but Fallout 3 would not.

Good point, although VATS gave you a sort-of replacemnent. Still not the same. Anyway, like I said, IMO, there were still enough similarities.

Well, we'll have to disagree on that. I know your definition of RPG is far narrower than mine.

I haven't played it much, but isn't there a huge leap between Red Faction and Red Faction 2 already, regarding the setting and the way how characters are presented? There are gameplay changes, too, like switching from save everywhere to checkpoints, and no destructible environment, IIRC, but I think I read about the connection between the settings of the two games being very loose, and the characters seem to have a wacky style almost reminiscent of anime, compared to the realistic looks they were going for in the first game.

So yeah, RF Guerilla turned a linear FPS series into open world TPS, but regarding the presentation, it might have more in common again with the first game than the second game had?

Well, you're half right: https://www.youtube.com/watch?v=wTDUuBWGtpU

(I can countenance still calling Fallout an RPG into 3/NV, but saying gameplay is similar in a shooter to an isometric tactical turn-based game? Bleh.)

Those I would consider to be two completely different series, just using the same franchise. But the King's Quest example is a good one, independently of the "natural" change from 2D to 3D. King's Quest VIII definitely has a whole new tone and more or less a switch in the genre, despite being part of the numbered series.