Viewers were stunned in August when it became apparent that NBC's Days of Our Lives had dumped the mega-popular Robert Kelker-Kelly who says he was ''bored'' by his part as rogue cop Bo and replaced him with Peter Reckell, the actor who originated the role. Reckell who played Bo from 1983 to 1987, and again from 1990 to 1992 last left the show to ''recharge his batteries'' and put in some time on the stage. But when ''the show called and said, 'Could you start next Monday?' '' Reckell recounts, ''it took me all of three seconds to say yes.'' Days had already gone from fifth to second among soaps a status that's certainly been helped by Reckell's return. If only the Nielsens would remain as lofty as Reckell's mood. ''My second day back, [costar] Lisa Rinna said, 'Peter, could you try to come down from the ceiling?' ''