Creating comic strips was and is a demanding task, making the artist a slave to his drawing board for years and sometimes decades on end. If a strip becomes successful in terms of the number of newspapers subscribing, the artist is likely to hire an assistant or two to do some of the grunt work such as drawing and inking backgrounds. In some cases, the artist might simply focus on creating plots, hiring another artist to "ghost" the images. For example, ace fantasy artist Frank Frazetta ghosted Al Capp's "Li'l Abner" strip for several years.
Then there is the case of the original artist abandoning the strip and another artist taking over. When Alex Raymond was killed in a car accident, his "Rip Kirby" strip was taken over by John Prentice who was skilled enough to maintain its general appearance. George Wunder replaced Milton Caniff on "Terry and the Pirates," and followed Caniff's pattern fairly well aside from drawing faces with oddly-shaped noses and other features.
An important artistic succession in terms of the the history of American comic strips and their appearance had to do with the "Scorchy Smith" aviation-related comic strip. Its creator, John Terry (brother of Paul Terry who created the Terrytoons animated cartoons) was dying of tuberculosis and had to abandon it. Its syndicator, the Associated Press, wanted to save the strip because it was fairly popular. So staff artist Noel Sickles was asked to take over.
Details regarding this along with many examples of Sickles' illustration work plus all his Scorchy Smith panels can be found in this outstanding book.
It seems that Terry could hardly draw and that Sickles was extremely skilled at depicting nearly everything. For the first few months of ghosting Scorchy (no one was sure if Terry could return to work, but assumed that he might), Sickles gritted his teeth and mimicked Terry's style, even signing Terry's name. Within a few months it became clear that Scorchy was now Sickles' strip, so he began a careful stylistic evolution away from Terry's crudely done panels to a bold style that influenced other comics artists working on strips dealing with real people as opposed to cartoon characters.
Below are a few panels showing Sickles' progression. In later years (he worked the strip for about three years), Sickles played around with other styles, though his core draftsmanship shone through.
Terry's work is so poorly done, I'm surprised that the strip survived at all.
When he had to, Sickles could imitate Terry pretty well.
By this point, Sickles is still signing Terry's name, but the images are much better. Note the female character who introduces herself as "Bunny." Sickles is including his pal Milton Caniff's wife Esther (who was usually called "Bunny") in this episode.
Sickles began signing his own name as of the April 2, 1934 panel. By May, we find a huge transformation from the Terry product. Note the varying perspectives and use of chiaroscuro brushwork replacing stage-type views and pen drawing. Caniff picked up this general style and applied it to Terry and the Pirates in masterly fashion.