“Buffy the Vampire Slayer,” arguably the WB Network’s biggest success ever, will leave that network after this season for rival UPN. (The WB, like PEOPLE, is part of AOL Time Warner.) “Buffy” star Sarah Michelle Gellar, 24, who has previously made public statements expressing a lack of interest in doing the show for any network but the WB, remains under contract to the program and will join it on UPN. She is currently in Australia filming “Scooby Doo,” costarring her fiancé, Freddie Prinze Jr., 25. The move of the Fox-produced show to UPN was because of money, with the network paying a license fee of $2.3 million per episode for the first season and then $2.35 million per for the second. WB had been paying a $1.1 million license fee, say reports.