[Spoilers ahead for Marvel comics and Being Human]
A series’ heart is its characters—whether it’s comedy, tragedy, fantasy, what have you, generally speaking, if you’re going to really capture the audience what you want is a good cast. You could have the most banal or wacky concept in the world, but if you have good characters people like and are interested in, people will watch it. Similarly, you could have the coolest and most fascinating backdrop ever, but without good characters to form that human connection, nothing’s going to glue. So, once you’ve got this band of characters that forms the bridge of audience attachment, you’d be silly to change them, right? Well, not always. Not every series revolves around the same set of fictional people for its entirety, and sometimes it’s beautiful and sometimes it’s bad.
Some series cling to their characters for decades, some change them every few seasons as a matter of course (like Skins), some bring back beloved concepts with new faces (Star Trek: Next Gen perhaps). Every long-running series has a kind of conceptual mould at its heart (e.g. Madoka Magica’s mould is “young girls fight monsters and discover the evil in the system they’re fighting for”) and a set of main characters (Madoka, Homura, Sayaka and co.). Sometimes, if they run long enough, these can get a little tired, so you have to change things up, unless you’ve got something truly episodic with no excessive continuity like old sitcoms. Generally, you can either change the characters (for example, bring in a new group of Magical Girls to follow) or break the mould (now instead of this being a story about fighting monsters it’s about fighting each other and their various dubious motivations).
Comics often keep their moulds, but get new characters within it. The new Thor comics star a woman (to the ecstatic cries of one half of the internet and the groans of the other, of course) not because Thor as we know him has been warped into a sex change, but because a new character has picked up the hammer and gained the powers therein, thus becoming the person to carry the title. So you can still have all your adventures that play with the universe and themes that suit that story, but to keep things fresh there’s a new lead to follow, get attached to, come to understand. It keeps the flavour and formula the same, but changes up the human connection to make things interesting and fresh. Thor was also a frog at one point, I’m pretty sure, so it’s not as if this is something new.
Captain America’s shield and title were actually held by Bucky Barnes for a time while Steve was a little bit dead, and now, due to yet more shenanigans, Sam Wilson has taken up the mask. This has happened for countless superheroes over the comics, and works well, I think, because again, it’s still Captain America and readers can rely on the same style and nuances of adventure they’ve come to trust, but when it comes to taking off the helmet you’re dealing with new guys which switches things up. Superheroes are all about what they represent, and it’s always interesting to see what new people bring to that in terms of the little things. You know they’re going to be a righteous symbolic badass because they’re Captain America, but you don’t know how that character in particular is going to present it.
So yes, you can quite safely change your main characters and keep popularity, as long as you keep the essence of the work itself the same. The mould of ‘superhero protects the world’ that you actually came to read about remains the same without any writer pulling the rug out from underneath you, but you get new characters within that framework to switch around and play with. Being Human is another example, keeping to its ‘vampire, werewolf and ghost trio share a house’ mould while eventually changing its entire main cast. The question can arise, though: are these new guys just paper cut-outs meant to fill the gap the original three we actually fell in love with left?
It comes down to fitting them into the formula without having them be straight-up replacements. I think each of the new group were individual enough that they didn’t feel like Mitchell 2.0 or what have you. This can happen though, especially when writers (or poor, befuddled casting agents having to deal with a star who’s left) feel they have to keep to the character format to keep the viewers attached, and bring in a new character to fill the role of the removed one rather than making them a character in their own right. This could be said to have happened with Pierce’s replacement character on Community after Chevy Chase left, who essentially fills the same archetype because they felt they needed a crappy old man in their cast.
This could also be argued to apply to Doctor Who wherein Clara is Amy 2.0 but with more mystery, who in turn was kind of River 2.0… the joke here is that Moffat knows the archetypes he likes for women and will damn well stick to them. Which is ironic considering that that’s a show whose core concept is the same main character with a new face every now and then, almost a perfect example of keeping to the mould while bringing in fresh characters (even if he’s the same guy every time, technically, each regeneration has different traits). It causes a little existential crisis at the start of every season, but otherwise the formula seems to work—no one actor is stuck with the job and can contract however they please, leaving the role open for new talent every few years so it stays interesting and we can all have our favourites.
Although, it’s still the same Doctor with all the same experiences, and those get more extensive and traumatic every time the show moves (whoops! Another genocide we’re responsible for) simply by virtue of it being an adventure story that has to up the ante every season, or just keep tension and conflict and stuff happening. And that can weigh characters down until they’ve literally been to Hell and back (hey, Supernatural, getting to you in a moment) and have so much trauma on their shoulders we really feel horrible watching them drag it all around. Again, I refer to Being Human, who actually let its characters rest in peace after everything they’d been through as opposed to putting them through an apocalypse as well and forcing them onwards trying to write their PTSD in the most believable way that didn’t impact the plot.
Supernatural—yes, I always come back to this one, since I can’t believe it’s been going on so long when it was planned to end at season five—is in its tenth season now, and the status quo of “two brothers road-tripping across America and investigating spooky stuff” has shifted. Since so much of that relies on Sam and Dean, it wasn’t really feasible to exchange them for new guys. So they’ve been through a ridiculous amount of stuff, and I think it’s safe to say they’ve gone for breaking the mould as Dean is now a demon. There’s only so much epic adventure you can put people (even if they’re fictional, they have to feel like people) through before a) they’re so wrecked by the experience they shouldn’t possibly be themselves anymore and b) you’re scraping the barrel as a writer coming up with new scenarios for your Sams and Deans.
Sometimes, characters need to be retired, and yes they will be missed, but sometimes it’s better to have the same concept with new faces than the same faces trudging through ten years of fantasy-generated awfulness and a depleting pool of ideas (or, you know, just end the show). Some shows and series successfully troupe through hundreds of episodes with the same cast and keep it interesting, some switch it up ahead of time. Either of these approaches can fail spectacularly, of course, but it’s always interesting to see.
As for which one I personally prefer, I’d have to say it depends on the characters in question. I miss the original Being Human trio, but I’d rather just rewatch the first seasons than put them through increasingly ridiculous ante-upping adventures, and leave their character arcs where they lie. I’m also totally up for drastic changes like lady Thor and black Captain America if they punch a progressive hole in perceptions while also showing they fit perfectly into the beloved status quo. I’d rather have a total mix-up like that than have the illusion of change like with the new Doctor, but still keep all that Moffatty goodness. It becomes a case of same crap, different smell.
So yes, please, switch it up if it serves your story. If you lose characters you like, they’ll always be in your heart.