Lea Michele

"Glee-steria."

That's the word Glee co-creator and executive producer Ian Brennan uses to describe the summer of 2011 when it was reported that stars Lea Michele, Chris Colfer and Cory Monteith would be among the graduating seniors and would not be returning Season 4.

"I was like, 'I can't believe this is such a huge deal,'" Brennan tells TVGuide.com. "We had to do a lot of reassuring to people: Not only are you not going to miss any of the characters you love this season, but we're probably going to follow them when they graduate. It was more of a nuisance than anything."

New Glee Season 4 posters: New York takes a bite out of Rachel! But where is Quinn?

Fourteen months later, (most) burning questions about what post-graduation Glee will look like will be answered when the Fox musical comedy returns with a slightly new format, new faces and a new location — New York City! "We put a lot of thought in it. We didn't take a hiatus. We kept writing through what would normally be a five-week break just to make sure we cracked the way we wanted to tell the story," Brennan says. "It was on our minds all last season and it's been on our minds the entire series."

The Season 4 premiere Thursday (9/8c, Fox) will be split between two narratives. There are new faces competing to join the newly crowned national champions New Directions while the club's former shining star Rachel Berry finds herself at the bottom of the superstar food chain — and on the receiving end of constant verbal assaults by NYADA dance teacher Cassandra July (a fiery Kate Hudson). "It just really started to click and then I realized, this isn't the first time we've done this — we've split the narrative before," Brennan says, referring to Kurt's tenure in blazers at Dalton Academy during Season 2. "It was fairly seamless, and I actually think it's the same thing here. Musicals just allow you a little bit more room creatively. ... We have the visual splash of being able to film in New York and, in my mind, when you watch that first episode, it expands the scope of the show in a very adult way."

10 Glee spoilers from Comic-Con: Goodbye Rory! Hello Sue's baby! And can Finchel survive?

But for a show that boasted 15 series regulars last year, why expand at all? For one, Brennan said the show's creative team wanted to avoid a M*A*S*H-like scenario, where the Korean War was stretched to 10 years instead of three. "You can't really tell a high school story without hitting high school milestones like homecoming and graduation and prom and your first kiss," Brennan says. "All those things need a passage of time."

Plus, the show wasn't ready to lose its most important characters. Instead, the writers wanted to explore how the graduating seniors would make good on Mr. Schue's (Matthew Morrison) whiteboard wisdom. "Rachel, like it or not, is really the core of the show," Brennan says. "She's sort of like the ultimate underdog and it felt weird to pull her out of that. She's part of this continuous story. ... You want to actually see, 'What does that mean once you leave that womb?' Rachel goes from being a big fish in a small pond to a very small fish in the biggest pond ever."

The hot summer songs Glee should do in Season 4

The writers are also freshening things up by adding several new characters. After all, there are six empty seats in the choir room that need to be filled by sectionals and new people that Rachel will encounter in the Big Apple. These new additions include Marley (Melissa Benoist), Jake (Jacob Artist), Kitty (Becca Tobin) and, at NYADA, Brody (Dean Geyer). "They just pop in a really interesting way, and I really think people are going to like them," Brennan says. "You get to see the glee club and the high school experience through fresh eyes. ... You're able to go back to basics."

But what will happen next season after another group of longtime glee clubbers, including Artie (Kevin McHale) and Tina (Jenna Ushkowitz), flip their tassles? Stay tuned. "We have a general idea of where we're going to land and what that last episode [of Season 4] is going to look like," Brennan says. However, he reassures fans (again), "The DNA of the show will never change."

The new season of Glee premieres Thursday at 9/8c on Fox.