Basic Trope: A long-running series changes over the course of its run.
Straight: When the TV show "Alice and Bob" first launched, the animation was cruder, most of the major characters were missing, Bob was kind of a jerk, and Alice was more of a wimp. Later, they developed into a Nice Guy and an Action Girl, respectively.
Exaggerated: Originally, the animation was terrible, Bob was rude, lazy, arrogant, and selfish, Alice wasn't even in the show, and Bob was a minor character in a show about his friend Charlie. Later the show started focusing more on Bob, who evolved into the epitome of Incorruptible Pure Pureness. Later Alice was introduced. She started out as a complete wimp who would burst into tears when she was startled. Eventually she became an Action Girl who could kill forty mooks with one blow. Meanwhile, the artwork improved dramatically, the voice actor who played Bob was fired and replaced by someone who sounded totally different, every character from the early episodes was scrapped except for Bob, and dozens of new characters were added.
Downplayed: Bob's personality was a little bit wooden, and Alice was more of a minor character. Eventually Bob developed a real personality, and Alice became Bob's sidekick.
Justified: People's personalities change over time, so it would be jarring for the characters to remain static, specially if important events are happening around them.