There's a lot that will seem different this weekend when the 40th season of "Saturday Night Live" premieres, with Chris Pratt as guest host. Many old faces and voices gone, some new ones just arrived, and even some on-air talent demoted But more than those cosmetic changes, what should we be expecting from the four-decade-old comedy show?

For one thing, we can expect the chaos and growing pains of last season to continue. That doesn't mean the show's in crisis; indeed, for a show that's been around so long, it's all but inevitable to muddle through periods of upheaval.

About once a decade, "SNL" seems to go through what losing sports teams call "a rebuilding year," scrapping much of the cast and starting from scratch with newbies. Unfortunately, "SNL" fans have to watch them learn on the job. We have to endure horrible seasons like the 1985-86 year or the 1994-95 stretch. But the results are worth it. After '86, the show gave us the Jon Lovitz-Dennis Miller-Dana Carvey years. After '95, the series came back with Will Ferrell, Molly Shannon, and Darrell Hammond. And in 2005, the producers added to an already strong cast two key players: future all-star Bill Hader and Andy Samberg, whose viral digital shorts really did put the 30-year-old show back on the cutting edge.

We seem to be in the midst of a similar painful renewal right now. In the last couple of years, the show has lost Samberg, Hader, the invaluable Kristen Wiig, Fred Armisen, Seth Meyers, and now, Nasim Pedrad (she quit after five years to join the cast of new Fox sitcom "Mulaney," produced by "SNL" guru Lorne Michaels). A lot of the newbies brought up last year didn't work out and have been sent back to the minors. (Sorry, fans of Noel Wells, Brooks Whelan, and John Milhiser.) Other failed experiments: putting writer Mike O'Brien on camera (he's going back behind the scenes) and making sophomore player Cecily Strong the new "Weekend Update" lead anchor (now she'll show up on "Update" in her old incarnation, as her drunken loudmouth know-it-all party girl character). And it's rumored that mainstay Kenan Thompson (at 12 years, the longest-tenured member of the current cast) will leave the show at the end of the coming season, though his departure has been rumored for years now.

There's only one new face (two if you count writer Michael Che, who left the "SNL" for a few months to work at "The Daily Show" and is now returning to claim Strong's on-air plum as "Weekend Update" lead anchor. Whatever they demoted Strong for, it wasn't inexperience). New featured player Pete Davidson, a New York-based stand-up comic, is a multiracial hyphenate who should prove versatile in sketches and in addressing the audience directly during "Update" bits. He's also 20, the first "SNL" player born in the 1990s.

One more subtle change: After the death last month of Don Pardo, who served as the show's announcer for 38 of its 39 seasons, the new announcer will be Hammond, the show's longest-serving cast member ever (15 seasons, from 1995 to 2010). During his tenure on "SNL," the impressionist comic once subbed for the ailing Pardo, but he'll be announcing now in his own voice. So there'll still be at least one connection to the show's institutional past heard on every episode.

That past, of course, is the show's greatest asset and its greatest burden. "SNL" has been around for so long now that it's become the sort of institution it used to mock. Anyone who grew up watching the show has their favorite era (there are about seven or eight to choose from), and it's now commonplace to complain that the show hasn't been funny or relevant since... well, whichever era was important to you.

Still, sometimes the show gives the culture what it needs, sometimes without realizing it's doing so. In 2005, Samberg and Youtube made each other famous; it took the show's pre-taped "Lazy Sunday" sketch to popularize the then-fledgling video streaming site (which in turn boosted the popularity of "SNL" for a new generation) and, more broadly, to popularize the concept of sharable viral videos. In 2008, filling a need the culture demanded, "SNL" brought Tina Fey back to play then-Vice Presidential candidate Sarah Palin, effectively redefining the popular image of the Alaska-bred politician for all time. Last year, with the country in the midst of the age of Michelle Obama, Beyonce, and Shonda Rhimes, the culture again made demands on "Saturday Night Live," which responded by hiring Sasheer Zamata mid-season so that someone other than Kenan Thompson in drag could play black women.

There may never be a resolution to the question of whether "SNL" should respond to the culture or try to lead it, but the show has largely been complacent in recent years about being merely reactive, not proactive. And even the political sketches are pretty toothless now. (Jay Pharoah has President Obama's speech patterns and mannerisms down, but his version of the president is usually a straight man reacting to the craziness around him. There's no sense about his performance, as there has been with previous "SNL" presidential impersonators, that his Obama's personality quirks are linked at all to his policies, accomplishments, or mistakes.) James Downey, the scribe who wrote "SNL"'s political sketches for decades, retired recently, but he acknowledged that he was becoming more conservative in his later years, which may have led him to pull some punches aimed at presidential authority. It's a lot easier to reflect consensus than to shape it, but "SNL" doesn't really even try to do the latter anymore.

"SNL"'s nostalgia for itself is likely to be on display throughout the season. Such "SNL" alumni as Hader and Sarah Silverman are scheduled to host over the next few weeks. (Sliverman was one of those odd talents that the show took pride in discovering but failed to nurture; she was gone after just one year in the early '90s. Today, she's established herself as a unique comic voice and a successful stand-up performer and TV mainstay, but it'll surely be bittersweet for her to return to the stage that launched her career and then unceremoniously booted her from the nest.) The "SNL" nostalgia fest will culminate with a three-hour primetime 40th anniversary special in February. (The actual 40th birthday is next October, but February is ratings sweeps month.)

Three hours of patting itself (and Lorne Michaels) on the back may sound a little bit much, especially for a show that, at this late date, is struggling once again to redefine itself. And yet, there's much to be said for the current incarnation of "Saturday Night Live." Its core players –- Thompson, Pharoah, Strong, Taran Killam, Aidy Bryant, Bobby Moynihan, Vanessa Bayer, and especially Emmy-nominated Kate McKinnon –- are as strong a group of sketch players as the show has had in years. Kyle Mooney and Beck Bennett seem to be off doing their own thing (much like Samberg in his early years), but they do seem to be cultivating a fanbase. Zamata has shown promise, and Davidson is surprisingly well-seasoned for a guy not yet old enough to drink. (As for Che and fellow "Update" anchor Colin Jost, let's just say they've got their work cut out for them.)

But what most explains the show's longevity and persistence isn't its storied history as a talent farm or its many moments in sketches that have become cultural touchstones. Rather, it's that magic word "live." Because of that word, hope continues to spring in the breast of the longtime-but-recently-disappointed fan. Because of that word, anything could happen, maybe something funny that we'd all still be sharing virally on Monday or watching in clip shows years from now. Because of that word, the show remains volatile, unpredictable, and yes, vividly, joyfully alive. The talent may or may not be there, the execution may be spotty, but maybe this weekend, it'll be the weekend that something happens, that the chemistry clicks and ignites, and that "Saturday Night Live" reminds us of why we used to care and maybe still should.