This isn't going to be a discussion of a single series, but a pattern in series. Series can, as hard as they try, just tread water, like "Monk," but most of the time, they choose to make a major change.
Romantic Changes
First, there's the change where they resolve "Will they or won't they?" Look at "Raising Hope," where eventually, the girl Jimmy has been chasing after the whole first season eventually starts dating him, or "Bones," where Bones and Booth get together and have a kid, or even Psych, where Juliet gets together with Shawn. They refuse to drag out the romantic plot, allowing it to evolve.
Switching out Cast
This is something that usually becomes necessary, but is rarely voluntary. When Monk gets a new personal assistant, it changed the show, but not fundamentally. When Monk got a new psychiatrist, it changed the show, but not fundamentally. This was because neither change was willing.
You look at shows like Being Human (UK), which cycles out nearly its entire cast, by series 4, and the changes in the show make it clear that it was prepared for, if not voluntary.
Plot-based Changes
The Riches and Fringe tend(ed) to evolve as an entire show, not just with character-relationships. The Riches sets up several issues in the pilot which could have lasted throughout the series: The stealing of the family's money, the arranged marriage De De is being forced into, and their pretending to be The Riches. Two out of the three of these issues get resolved even before the first season is up, paving the way for a new antagonist that would have never been expected, and an evolution of an old antagonist.
Fringe, on the other hand, seems to build season-long arcs, and sticks with them.
Season 1 was very procedural, slowly revealing a grand plan, which is followed up on in season 2, though Season 2 becomes inherently less episodic.
Season 3 throws the grand plan out entirely, dealing instead with the two alternate realities, and the two versions of the main character.
Season 4 deletes, then brings back, a major character of the cast.
Each season is fundamentally different from the previous one.
Reboots (and rotating cast)
Sometimes a show will reboot itself. This is usually by design, and rarely out of need. Examples of this are: Every reality tv show ever, Skins, and American Horror Story. Reality TV shows have to; it's part of their design. Each season gets a brand new cast, with new challenges (so they can't cheat off of the old ones in game-based ones).
Skins and American Horror Story, on the other hand, make it an artistic choice. For Skins, it seems to be a character reason. For American Horror Story, it seems to be a tonal reason. See, in Skins, each episode is a character study. There tends to be a few episodes about everybody (first and last of the season, typically), to make the connections. At the end of two seasons, they create a new cast.
American Horror Story, on the other hand, seems to understand horror. They understand that it is unknown, but prevalent. Because of this, each season is a total reboot of the series. The first season starts in a haunted house, the second in an asylum. Giving each setting only a season allows it to prevent us from becoming too familiar with the horrors, and prevents it from repeating itself.
This is very rare, as people usually identify with characters. It's hard for people to get behind the idea of a show, rather than the characters of a show.
Shows in Stasis
It's strange that when I looked back through the shows I had watched, there was only one show that leapt out at me as one that never changed. I've mentioned it a few times now, and that's "Monk." Why do I say that? It changed, didn't it?
Now, metaphorical audience member, answer me this.
Did Monk ever (permanently)
Change his job?
Change his house?
Change as a character?
Sure, some of the people around him changed, but Monk himself never changed. The show was afraid to take any risk. When he got a new house, he immediately gives it up. When he goes on meds, he immediately gives that up. Not because they don't work, but because they interfere with his job. When he gets the job that he was going after for 9 years, he isn't happy and quits. The show loved the status quo.
Some of these are understandable, at least, from a writing standpoint. Monk on meds? He wouldn't be himself! But others seemed to be based on a fear of change. They didn't want to write Monk the police detective. They wanted to write Monk the P.I. That's fine--but it can't last forever.
Conclusion
I'm all for shows that evolve. When it goes wrong, it can go horribly wrong, but if the show never takes the chance, the audience may very well get frustrated, and feel like their beloved characters are treading in water. If the male and female lead have had unresolved sexual tension for 14 years, will we still care? Maybe the resolution just wasn't worth sticking around for.
