When you have a series of games and the genre changes between them, do you think of it as a good thing or a bad thing?

One example of a genre-changing series is Konami's Knightmare series (on the MSX):
* Knightmare 1 is a shoot-em-up, albeit with fantasy theme.
* Knightmare 2 is Maze of Galious. (For anyone not familiar, this game is a 2D platformer that's structured like the original Zelda, with a main world and some dungeons.) This game ended up influencing some modern indies, including La Mulana and Unepic.
* Knightmare 3: Shalom is an adventure game with a JRPG style interface (but no JRPG combat).

On the other hand, we have other series that just stick to one genre throughout.

Do you think it's good when a series changes genre, or would you prefer it if the series would stick to one genre (which would prevent the series from losing fans due to genre switches, and would mean that you gave a good idea of what a game is like based off earlier games in the series)?

Yoshi's Safari. Going from a platformer to a FPS was a strange, and given the design issues of the Super Scope, unwise choice.

I think it's good when franchises experiment with genres, but they should do so with care. Super Mario RPG wasn't a slapdash effort made by some no-name developers in 3 weeks.

Going from one genre to another in the course of a series is fine by me (i'll just avoid it and be on my way!), even if it means a favourite racing game turns out to be a horror game, for example what the successor of NASCAR Racing 2003 Season came to be after EA took over the franchise.

What i don't usually enjoy is when the genre changes mid-game / end-game, for example what Brütal Legend did when it turned a fine action-adventure game to an RTS-like game near the end (and i'm fine with RTS games mind you!). Being a metalhead myself is the reason i endured through it, otherwise i'd have dropped it by the time the whole RTS thing started.

Also, Divinity series did a 180° turn multiple times, from Hack & Slash to aRPG to TBS to cRPG (to, who knows, a Flight Sim next?), though Larian gets a free pass from me cause i like those guys. :P

Depends. I liked it with Castlevania and Wonder Boy, but at the same time it's important to me that it still fits the actual game.

I don't think combat is a requirement for (J)RPGs.
Most games of that genre do have that, but you can have all kinds of stats and experience even without combat. Personally I would in most cases prefer games to go from RPGs toward adventures, because raising stats and all that isn't very rewarding in the long run, with exceptions of course. It depends, but sticking to one genre isn't going to guarantee anything about how the games are like.
Think about King's Quest. Some games have parser, some are point-and-click, the reboot is partially QTEs, sometimes you play as a man, sometimes as a woman, sometimes it's pixelarts, sometimes it's cute cartoon graphics, sometimes it attempts to look realistic. But it's all within adventure genre anyway.

The only time that the series possibly jumped genre is with Mask of Eternity, which sure is different, but the other parts aren't alike either, like said. Players have debated for decades whether MoE is adventure or not, and whether it should be counted as part of the series, or not.

I suppose if it works, don't fix it is a good advice, but that also means that series would get very repetitive very soon. Some new ideas are OK. If it takes the game to a different genre sometimes, I don't mind it that much, if the game is good.

There are some cases where going to different genres has worked well, like Police Quest and SWAT. I think the spin-off series is almost better known these days than it's adventure origins.

I disagree; I enjoy raising stats.

In fact, this is why I've enjoyed the two idle clickers that I've played, where getting numbers to increase is the whole point of the game (and at least Cookie Clicker gets to the octillions and beyond). Regardless of how you define RPG, it's pretty clear that it's a different genre. Also, I believe Shalom doesn't even have stats or equipment; it just happens to borrow the world map aspect from the RPGs of that era (though note that the world is split into separate screens like in Zelda 1 or Maze of Galious because of hardware limitations).

Prey and its expansion Mooncrash. Not exactly a series, but I feel it's worth mentioning because it doesn't happen often that expansions are a different genre than the main game.
https://www.gog.com/forum/general/what_game_s_are_you_playing_right_now/post9831

I would say:
No to the first question.
Yes to the second.

Inventing a genre and changing to a genre are two separate things.

If Mario 64 had been the first Mario game, and all later Mario games had been in that genre, then the game might have still invented the genre, but it wouldn't have changed genre at any point.

(Incidentally, Mario 64 is different enough that I tend to regard it, and the games that followed in that style, as a side series rathre than as part of the main series that SMB1 started.)

I have played so many vanilla (in the other sense) Doom II megawads I'd be fine whit a series churning out a whole lot of the same, if I rather enjoyed the core gameplay. But great games, like Wizardry 8, have come from experimenting with the formula or drastically altering the core gameplay. Of course spin-offs and side games are great for experimentation.

Hmm, I think changing genre mid game can be fun, so many games love having a shmup section for some reason. Didn't the first six Kirby platformers, more or less, do that?

Isn't Super Mario Bros. essentially a spin-off of the franchise that began with Donkey Kong?

I'm still conflicted on rather I dislike collectathons or just Rare's 3D platformers. I mean my favorite 3D platformers are Ape Escape, A Hat in Time, and Sonic Robo Blast 2.

Really just depends on if I like the genre it switches to. Fallout being a good, obvious example. I like FPS games, so switching such a great world and aesthetic to that genre worked for me just fine. On the other hand I hate MMOs, so Elder Scrolls going MMO only for a decade after Skyrim was pretty frustrating. Same for Fallout 76.

Surprised nobody mentioned Divine Divinity yet.
Changed from thinking man's Diablo to 3d action-rpg, to some weird mix in Dragon Commander, to turn based Original Sin 1 and 2.

Don't forget Super Mario Land.

Then again, at least these games stay within the realm of an action game of some sort; a sudden shmup like sequence in a game like Ultima, Wizardry, or Dragon Quest would not be appreciated by many players (and it creates an accessibility issue).

Side note: It turns out that Celeste actually has a mod called Chapter 6 EX-side which turns the game into a bullet hell, though that's perhaps more pertinent to the other topic I recently made.
At least TES Online and Fallout 76 are named as side story games, unlike Final Fantasy 11/14 and Dragon Quest 10 (which should have been called Final Fantasy Online (2) and Dragon Quest Online).

Also, did you have any thoughts about Ultima going MMO with Ultima Online?

In theory I don't mind as long as the game is good. But I can't think of a positive example at the momet...

Change of genre in X-Com Interceptoe and Enforcer was a poor choice

Barkley Shut Up and Jam! (1993, Accolade) - Sports, 2v2 basketball.

Barkley, Shut Up and Jam: Gaiden (2008, Tales of Game's Studios) - JRPG

Barkley 2 (2079, Tales of Game's Studios) - Action Vaporware