After Megatron was badly injured and thrown out into space by Starscream. He encountered Unicron who gave him more power and was reborn as Galvatron.
After his fight with Optimus Prime, the evil leader Megatron had emerged victorious and killed his arch nemesis, but was severely damaged, later Starscream ordered the wounded Decepticons to be thrown into space, and personally threw Megatron out. While floating in the vacuum of space Unicron takes Megatron's nearly dead body and rebuilds him into Galvatron.
Info coming soon
Beast Wars II
Beast Wars II Galvatron
It is unclear whether or not this Galvatron was linked to the original Megatron or to Megatron 2. Given the point in time that he existed it is entirely possible he is neither of the two. In Beast Wars II the character was always called Galvatron and never referred to as Megatron. This Galvatron draws his power from an object called the Galva-Matrix.
Megatron is upgraded into Galvatron towards the end of the series.
Energon: After seemingly having died at the end of Armada, Galvatron returned butwas once again simply called Megatron, though he would be known as Galvatron yet again towards the end of this series.
Cybertron: Started this series off as Megatron yet again, and of course powered up to bacome Galvatron towards the end of the series.
Has not appeared yet, but after the Megatron of this series put dark enrgon into his spark chamber he claimed to be able to hear the will of Unicron, so it may only be a matter of time before the Galvatron persona takes over Megatron in this series.