Bakuman has a drastic change in style (although not as drastic as other series). It is not really clear in the images, you have to see chapter one and the latest chapter yourself. :amuse
I think Obata-sensei was trying to transition out of the realistic art style of Death Note. (I don't count Ral Grad.) Despite the fact that Bakuman has a completely different story and theme, people will still try to compare it to DN because the author is also the same. By changing the art, the readers will read it as a separate manga.
If you look at this colour page he made for the Bakuman compilation book, he still has the ability to switch back between art styles. I consider the change in Bakuman a unique case of Art Evolution. :)
One of the most extreme examples that comes to mind is Vinland Saga. It might be partly due to Yukimara's moving from a weekly to a monthly magazine, but damn did his art improve as the series went on. I mean, the amount of detail he puts into it now compared when the series started is ridiculous.
When i read Vinland Saga from chapter one to the latest chapter i completely didn't realize the art evolution at all. I guess that is because the storyline is too engaging. :D
The change is very impressive indeed.
P.S. i am very grateful it change publication from a shounen magazine to seinen magazine.
I hadn't noticed either :XD
Looking back I can see a huge difference when he switched to Afternoon. That was a demographic switch too.
I haven't read any of Toriyama's work (*gasp*), but I can sense the change in his artstyle over his career. I also like the cartoon-y style of Dr. Slump the best.
I haven't bothered with DBZ, the evolution away from the cartoony, fun style of his early work and the change in tone to fighting manga that went with it.
I was reading the Wikipedia article for Vinland Saga, and they made a nice comparison of cover arts between when it was weekly and when it became monthly:
Bad image quality sadly, perhaps I'll add the full size covers later.
One Piece Art Evolution
One Piece has had a drastic change in artwork.
Chapter 43 of One Piece. Look at how "childlike" Luffy and Nami (in my opinion) look. Now, look at Zoro and Nami on that page and Luffy on the next page; this is from chapter 503 by the way. See the change in art? Luffy and Nami look more mature. In my opinion Oda changed his artwork to show the characters maturing and getting older.
Now, after the timeskip (two years), lookie: Chapter 602. don't they look more mature, even if the change's not that significant?
Oda's artwork definitely has changed over the years, as has Kishimoto's (though Part I was the same, but Part II had different style of art and changed throughout the chapters).
While reading OP, I actually never noticed the change since it happened gradually. It's much more evident now that the Strawhats from the different arcs are placed side-to side. The inking becomes thicker and more defined towards the recent chapters.
Not that major compared to OP but I put my vote on Kuroko no Basket.
This was the manga where I see not only change but DRASTIC improvement in drawing at the newcoming young sensei evolves in the same series in 2 short years.
The first chapters the drawing are not very good, and for simplicity style (or lack of good assistant & thus time) many features are omitted; like even the eyes are omitted at times (drawn on one side only), and lower legs shaded black.
And for a basketball manga..... it actually only introduces its Seirin teammate cast (of 6!) in chapter 8...... The fifth chapter into the first match! People deduced that the sensei much prefers to get story going and always forget about adding details back then.
The arts picked up quickly and by chapter 25 it's quite reasonable even at high bar or true sports manga.
And I remember I first heard 2ch-ers comparing its art to have become a little like Slam Dunk, around chapter 67-70.
Pictures I wish to provide later... ^_^;