MTV released a statement last week -- following news Sitch was seeking substance abuse treatment -- claiming the network expects the entire cast to return for Season 6 ... but sources close to 495 Productions tell TMZ, the production company is currently looking for new talent.

According to sources, 495 desperately wants to keep things fresh -- and folding in new castmates will help transition the show into a new generation ... while still keeping old fans hooked.

We're told 495 plans to limit Snooki and The Situation's roles on the new season -- giving new cast members more screen time instead.