Speaking on Ryan Seacrest's radio show on Friday, Murphy said that the upcoming Season 3 will be senior year for the original cast.
"I don't think of it in terms of eliminating or replacing. Because I think the thing about this cast is people love them and they are incredibly talented. They've left sort of an indelible mark," he said.
"The thing that I wanted to do and the cast wanted to do, we didn't want to have a show where they were in high school for eight years. We really wanted it to be true to that experience. We thought it would be really cool if we were true to the timeline."
Murphy revealed that Matthew Morrison and Jane Lynch will stay with the show beyond Season 3, with their characters Will Schuester and Sue Sylvester acting as "the male and female lynchpins of the series."
The empty spots will be filled by newbies who'll be seeded into the show throughout next season -- casting calls have already gone out for an "Adele-meets-Susan-Boyle" type among others -- including the winner of spin-off competition show 'The Glee Project,' who'll get a seven-episode story arc in the fall.
Summing up the revolving door policy at 'Glee,' Murphy said "I think the fun thing about the show is it's a celebration of youth and talent and I think that just like with the original cast, I think finding those young unknown people and giving them an opportunity to break into the business and become stars is a really fun and exciting thing and is the spirit of the series."
'Glee' Season 3 is due to premiere in September on Fox.