Douglas Fowley portrayed "Doc Holliday" in Season 2 and Season 3, up to and including Episode 12 of Season 3 ("The Good and Perfect Gift"). Myron Healey took up the role in Episode 34 of Season 3 ("Doc Holliday Rewrites History"). Douglas Fowley returned to the role in Episode 3 of Season 5 ("Tombstone").
This is apparently the first episode where Douglas Fowley takes over the role of Doc Holliday. Previously in the series he was portrayed by Myron Healey. In earlier episodes, Fowley played the older character of Doc Fabrique.