Original: In that episode, Frasier, on a trip to Boston, meets the Cheers gang (though not at Cheers itself) and Cliff thinks Frasier has flown out specifically for his (Cliff's) retirement party, which Frasier ends up attending. Frasier was on the air for as many seasons and almost as many episodes as Cheers, going off the air in 2004 after an eleven-season run.
Simplified: Frasier got his own show (Frasier) the season after Cheers ended.