On Law & Order, Jerry Orbach played a defense attorney (while Paul Sorvino was the senior detective) and came back the next season as Detective Lennie Briscoe.
Also on L&O, S. Epatha Merkerson played the mother of a defendant in season one before coming back as Lt. Van Buren. (In the same episode, ironically, James McDaniel played a crime boss before he, too, became a police lieutenant on NYPD Blue.)
On Cheers, Paul Willson had appearances as Tom and Glen (singing the words to the Bonanza theme) before the recurring role of bar regular, who always complained the gang left him out of things, Paul Krapence.