I think this occurs in all manga - the art just gets better as the mangaka gets used to drawing the characters, gets into the groove of things, gets experience. I mean we're talking about years here, it's only natural that the art will improve, and could look like an entirely different person was drawing them.
I'm always impressed when I compare Bleach's old artwork to its current artwork. It's a big improvement, and I really like the style with which Kubo draws his characters now. His proportion is a lot better now, for one. Improvements can definitely also be seen in Psyren, Nurarihyon no Mago, One Piece, Naruto...
And why am I naming only Jump series :-_- I obviously need to read more
Like mentioned before, another big evolution (or should I say evolutionS) would be the art from D.Gray-Man. It's like when Hoshino came back from an injury, she'd have a new art style :XD Personally, I love the art from the ArK Arc the best. Her current one is not too bad either, but somehow it makes certain characters look similar to each other, which imo is a bad thing.
Another interesting thing when it comes to manga art might be CLAMP's art style. For those who don't know that CLAMP is actually a group of people, they might be quite bewildered by the different evolving art styles that seem to pop up in their various manga. The style for say Kobato is very similar to Tsubasa Reservoir Chronicle and xxxHolic (and might as well throw Code Geass into this as well, lol), with long slender limbs etc. On the other hand, X/1999 and Tokyo Babylon share the same artstyle (same world after all) which looks more realistic and less exaggerated. It always depends on which of the CLAMP group are mainly in charge of drawing the characters or backgrounds etc, and this typically changes from manga to manga depending on the mood etc of the manga they want to portray. I thought this was a pretty interesting way of doing things :)