In both syndication and daytime network airings, the episodes' tag sequences were often cut.
The syndicated version called "Happy Days Again" used an alternate version of Bill Haley's "Rock Around the Clock" theme song. The original series used a specially-recorded version, but "Happy Days Again" used Haley's original 1954 recording.
The series' original syndication run was known as "Happy Days Again" from the late 1970's to mid 1980's. This was done to distinguish the reruns from the first run episodes aired on ABC at the time. The series' was reissued in the late 1980's-early 1990's as "Happy Days".
Two versions of the episode that introduced Robin Williams as Mork from Ork were broadcast. Initally, the episode ended with Ritchie waking up from a dream and seeing Robin Williams as an earthling. When the episode was rerun, this was replaced by a scene tying the episode into the new "Mork and Mindy" (1978) TV series. Most syndicated prints do not have the Mork and Mindy ending.