It's almost certain that you'll see your breakout faves again.
It's hard to imagine the sequel without Lana Condor and Noah Centineo, although neither of them have actually confirmed returning. Condor previously expressed interest in making a second movie and said that it was very likely that a sequel was going to happen. In early November, the actress mentioned that both she and Centineo had busy schedules, telling Entertainment Tonight, "[T]hey better hop on it pretty soon because Noah and I ‚ I'm on the show [Deadly Class]. I'm doing a movie in Romania. He's doing Charlie's Angels. He's doing another movie that he just booked. It comes down to availability, and I hope they lock us in soon."
As for the other cast, we'll likely see more of the Song sisters, Margot (Janel Parrish) and Kitty (Anna Cathcart), as they advise Lara Jean. In the book, Dr. Covey gets a love interest, so John Corbett is also likely a returning player. Josh has a smaller role in the later books, but we wouldn't be surprised if the filmmakers find a way to squeeze in Israel Broussard.