The very first Christmas-themed episode of 'Happy Days' featured a plot that had Fonzie alone at Christmas and Richie's efforts to invite him over. This was in 1974, an early episode that actually featured Chuck Cunningham. Years later, the episode was re-worked, this time as a flashback that Richie was relating to newcomer Al Delvecchio (played by Al Molinari). I saw the re-worked episode once on broadcast TV (I think in 77, long after Chuck was ret-goned) but all future broadcasts in reruns just had the original un-altered ep. I've tried to find out if any Happy Days home video releases include these wrap-around segments, to no success. Anyone know?
Edit: According to Christmas Specials Wiki, these extra scenes are sometimes seen in syndication, but not on the DVD sets. Which may make them more solid or more iffy, since by definition fewer outlets run Happy Days reruns nowadays.