Last week, when Matt Siegel promised to make a “big announcement’’ Monday, some listeners speculated that KISS 108’s morning man was finally hanging it up after three decades. “Charles [Laquidara] retired at this point in his career, and I think some people thought I was, too,’’ said the highly rated Siegel, referring to the former longtime WBCN DJ. “I’m going to do this until they throw me out.’’ So what’s the big announcement? Clear Channel has overhauled the lineup of WXKS-AM (1200), ditching conservative talk in favor of “Matty’s Comedy 1200,’’ a 24-hour comedy station hosted by Siegel. (He will continue to appear daily on KISS 108.) The new station, which launched Monday, will play taped comedy tracks by the likes of Dane Cook, Louis CK, Chris Rock, Jerry Seinfeld, Jeff Foxworthy, Daniel Tosh, and Eddie Murphy. Siegel will be a presence on the station, but he said much of contribution will be prerecorded intros and previously aired interviews. (Clear Channel is experimenting with comedy radio stations in a few other cities, too.) Of course, “Matty’s Comedy 1200’’ is one of two new stations that launched Monday. The other is RadioBDC, Boston.com’s radio station streaming alternative music, news, and contests, and featuring former WFNX personalities Henry Santoro, Julie Kramer, Adam 12, and Paul Driscoll.