This will be a tough folks.
But I'm still curious.

Is there manga authored and illustrated by the same mangaka, yet some character designs are more different than others.
In animation, this is more easier like Kazuya Kuroda's several works, each not the same designs.

Can anyone help me scout for such manga.

Please leave questions if you are confused.

For now, I've explained all I can.

Maybe CLAMP? Their later works (from 2000 onwards) have a very different style when compared to their earlier manga.

I second this. CLAMP has changed drawing artists over the years. The art of Chobits is different from any other series. :]

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In Hayate no Gotoku chapter 270 they mentioned a series named "Yuke!! Nangoku Ice Hockey-bu"(Southern Ice Hockey Club) with character's design completely different at the end than it was at the beginning. I don't really know about this though.

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its cold down here fam ~

I'll just say Id
It's the only work of the artist (as far as mu told me), but if you compare the beginning and the end, you'll be amazed. The change is incredible, seriously.

may i suggest Ah! my goddess
it's just one manga, made by the same artist..
and for the different style problem.... once you read along you'll definitely feel the change on it's drawing style.. from it's 80ies manga style to the current manga style... you might like it
i know it's not exactly what your looking for.. but it's worth to suggest

nakamura asumiko`s work!

D.Gray-man -- Jesus, what does Hoshino think she's doing changing the style every ten chapters? The art for the first eight volumes or so is pretty consistient and personally, it was my favorite style since it was all realistic and victorian, but how she's gone all modern-pop-ish.

for series: Gantz

for manga-ka: Matsumoto Taiyo, changes his style for almost every manga he does.

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Sora no Otoshimono - Over time the main character is represented as a chibi more often, and then, almost all the time.

Umi no Misaki - The character art became less detailed as the series goes on. Well... at least for their faces.

I just remember these because this part about them particularly annoyed me...

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I care waaaaay too much...