According to Omega Supreme, the Constructicons existed on Cybertron many years before the Autobots and Decepticons reached Earth. At first they were peaceful, till Megatron used the Robo-Smasher to make them evil. This is the second of three contradictory versions of the Constructicons' origin. The other two versions are seen in Heavy Metal War (1984) and Five Faces of Darkness: Part 4 (1986).
According to both Omega Supreme and Optimus Prime, the reprogramming effects of Megatron's Robosmasher can never be reversed, but in The Core (1985), Wheeljack and Chip Chase were able to sway the Constructicons to the Autobots' side with dominator disks.
Early in the story Megatron can be seen in the Decepticons' underwater base. Standing beside him is Sideswipe. How could Sideswipe be there when he is an Autobot?