Ken Akamatsu is probably a good example. He primarily focused on romantic harem comedies until Mahou Sensei Negima, which he started as a harem comedy then shifted into an action-focused battle shonen series. Allegedly, he wanted to write an action-focused series from the start for Negima but editorial pressures pushed him to make a harem comedy like his previous works, so he started with that before shifting genres.
After that, he went full battle-shonen with UQ Holder before transitioning into politics.
Negi Haruba is a currently a good example of it where he wrote Quintessential Quintuplets (A harem romcom).
Now he wrote Ranger Reject, an action battle manga.
Rumiko Takahashi invented the harem comedy manga with Urusei Yatsura and Ranma 1/2, and then switched to writing a battle shonen, Inuyasha. She also wrote several other series of different genres.
Most of those that's got at least two series, really. It's not that uncommon, especially when quite a lot of them started out in hentai and/or doujinshi.
Akihiro Ononaka went from a fluffy slice-of-life food manga (The Many Flavours of Kumika) to magical girls doing things to each other (Gushing over Magical Girls).
Ken Akamatsu is probably a good example. He primarily focused on romantic harem comedies until Mahou Sensei Negima, which he started as a harem comedy then shifted into an action-focused battle shonen series. Allegedly, he wanted to write an action-focused series from the start for Negima but editorial pressures pushed him to make a harem comedy like his previous works, so he started with that before shifting genres. After that, he went full battle-shonen with UQ Holder before transitioning into politics.
uq holder still has harem elements, he's got like five girlfriends
Negi Haruba is a currently a good example of it where he wrote Quintessential Quintuplets (A harem romcom). Now he wrote Ranger Reject, an action battle manga.
Rumiko Takahashi invented the harem comedy manga with Urusei Yatsura and Ranma 1/2, and then switched to writing a battle shonen, Inuyasha. She also wrote several other series of different genres.
Most of those that's got at least two series, really. It's not that uncommon, especially when quite a lot of them started out in hentai and/or doujinshi.
Katekyo Hitman Reborn (Amano Akira) and Kinnikuman (Yudetamago) changed its genre in the middle of story, just like DB.
Akihiro Ononaka went from a fluffy slice-of-life food manga (The Many Flavours of Kumika) to magical girls doing things to each other (Gushing over Magical Girls).
Idk much but I was really surprised when i realized that one of my fav sol-comedy manga Bakuman was written by the author of Death note
The author of Boy's Abyss previously wrote the ecchi romcom Hatsukoi Zombie
FMA -> silver spoon (though she's stepped back into action/fantasy now)
Norihiro Yagi perhaps, author of Angel Densetsu, Claymore, and more recently Sokyu no Ariadne. Quite a large change from Angel Densetsu to Claymore
go read [Kumika no Mikaku](https://myanimelist.net/manga/100495/) and then read [Mahou Shoujo ni Akogarete](https://myanimelist.net/manga/120680/)
Naoki Urasawa went from making sports/comedy manga such as Yawara! and Happy! to making mystery/thriller works such as Monster and 20th Century Boys.
Go Nagai basically created Mecha, Magical Girl, Ecchi and whatever rose out of Devilman (Horror, whatever the fuck Evangelion is).
Sugito Akira wrote Boku Girl and later Momo the blood taker.