Expect changes when The Hills and The City start up next month.

Beginning September 29, on the new night of Tuesday, the shows air back-to-back at 10 and 10:30 p.m. And the biggest change of all will be the former's principal character, with Kristin Cavallari replacing Lauren Conrad.

“Kristin has brought new life,” executive producer Liz Gateley tells People of the Laguna Beach star. “She is the same Kristin we know and love - not afraid to mix things up. Her presence has really changed every dynamic.”

In Southern California, Kristin Cavallari is ready to shake up The Hills, while Whitney Port will be anchoring a revamped version of The City on the East Coast.

Three thousand miles away, Whitney Port, who left The Hills for her spinoff The City, will be dealing with a new boss in what producers admit is a complete reinvention of the show after its lackluster first season.

Gateley adds: “A lot has changed in The City. We shifted the focus of the show significantly from a relationship drama to a workplace drama. [Whitney Port] is focused a lot more on her work now that she’s single."

"Kelly Cutrone (Whitney's boss) takes on a much bigger role in the show. The show feels much more like a reality Devil Wears Prada. Our whole shooting style changed. You’ll see much more action and less sitting and talking."

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.