With “Cheers” destined to end its run after this season, NBC reportedly is mulling a near-term shift of “Seinfeld” to Thursday, trying to boost the Castle Rock Entertainment comedy as a potential replacement for “Cheers” next fall.Move would reportedly put “Seinfeld” in the 9:30 p.m. slot. Paramount Network TV has been locked into that post-”Cheers” period with companion sitcom “Wings” but may have lost its leverage in terms of that time period now that the “Cheers” gang is closing up shop. In addition, “Seinfeld” is a critical and network favorite that’s been getting murdered in its Wednesday slot by ABC’s “Home Improvement.” NBC said it couldn’t confirm any scheduling changes, but sources say such a shift has been batted around. If “Seinfeld” does move to Thursday this season, the question is whether NBC would flop “Wings” into “Seinfeld’s” Wednesday slot, leaving that show and new comedy “Mad About You”–designed as a “Seinfeld” companion–to duke it out with “Home Improvement” and “Coach.” “Seinfeld” performs well in key younger demographics but some observers feel it doesn’t possess the same broad appeal as “Cheers.” Still, the assumption is that the show may have greater long-term potential than “Wings” and eventually has to move away from “Improvement.” With that in mind, NBC might be inclined to make the switch now while it can still capitalize on the “Cheers” audience. NBC scheduled “Seinfeld” behind “Cheers” for two months when the show premiered in May 1990 and again from April through June a year later to give the series extra exposure. This season, “Seinfeld” is averaging an 11.7 rating, 18 share Wednesdays at 9 p.m.