"Conquest of the Earth" is comprised of episodes of the television series, edited together into a two-hour film for theatrical release. The episodes are "Galactica Discovers Earth" (parts 1-3), and "The Night the Cylons Landed" (parts 1 & 2).
In Galactica 1980 (1980) (the TV series from which this movie is pieced together) the character Dr. Zee was played by Robbie Rist in the pilot, but was subsequently replaced by James Patrick Stuart. However, footage of both actors turns up in this movie. To get around this continuity issue, some lines have been re-dubbed to split Dr. Zee into two separate characters. Rist remains Dr. Zee, whilst Stuart becomes "Dr. Zen".
The footage of Earth destroyed by Cylons is actually from the film Earthquake (1974) with some Cylon fighters superimposed.