Great Transformation

The Great Reformatting is an event from the Beast Machines cartoon portion of the Generation 1 continuity family.

Cybertron now has better feng shui!

The Great Reformatting was a planet-wide reformatting of Cybertron itself, initiated by Optimus Primal, as both he and his opponent, Megatron plummeted into Cybertron's technorganic core. It marked the end of Megatron's rule over Cybertron, and of the battle for the sparks. In addition to transforming the planet itself into a technorganic paradise in mere moments, the reformatting caused the release of the sparks that Megatron had held captive for the duration of his rule, allowing them to return to their bodies. Individuals themselves were not affected by the event, and had to be reformatted individually afterward if they so chose to be.

Fiction

Beast Machines cartoon

The planet was reformatted when Optimus Primal had flung himself and Megatron into its organic core. As a result, Cybertron gained not only a green hue, but also gained biology not present before its reformation, such as trees. Endgame Pt. III: Seeds of the Future

3H comics

The Quintessons believed themselves to be in control of said reformatting, and had planned to conquer Cybertron subsequently. Their assumption was that the Transformers inhabiting the planet would be disoriented from this sudden change, however it was revealed that Primus had predestined the whole event, and Cybertron's populace were able to repel the invaders. Primus directly thanked Judge Heirodyus, as a representative of the Quintessons, for unwittingly assisting him during this evolutionary crisis. Wreckers: Finale Part II

Wings Universe

Time-traveling to the future when Cybertron would theoretically be at peace, unprepared for war, and rich in energy, Captain Cannonball of the Star Seekers led his crew in his greatest heist idea ever: use Unicron's head to drain all the energon out of Cybertron. This attracted the attention of the elite defenders of Cybertron in 2984, the Cybertronian Knights, and a battle ensued aboard Unicron's head. The battle was prematurely ended, however, when Alpha Trizer revealed to Cannonball that his scheme was doomed to fail because Cybertron in its modern state was technorganic and no longer ran on energon (as a result of the Great Reformatting). Hoist the Flag

Ask Vector Prime

Vector Prime stated that while a momentous occasion, his presence was not needed at the Great Reformatting. Ask Vector Prime, 2015/06/04

Notes

Name Origin

The name "Great Reformatting" has never actually been used in any official Transformers media. Rather, it's simply what the fandom has used to refer to the event for all these years.

The Beast Machines episode in which it occurred never directly gave it a name, while the 3H fiction simply referred to it by such informal descriptors as "the reformatting of Cybertron", "the recently reformatted techno-organic Cybertron", "the reformatting of the planet", "the planet's reformatting", and just "the reformatting". Likewise, Alpha Trizer's profile found in the Diamond Edition release of "Hoist the Flag" simply referred to it as "the reformation of Cybertron into a techno-organic world", while the comic story itself also made no mention of any name for the event.

The closest that the name "Great Reformatting" has come to having been used in official media to refer to this event was when it was included in the question that brought about the above-paraphrased Ask Vector Prime answer. However, since the question that used the name originated from the fandom side of things, the official nature of this question's use of the name "Great Reformatting" is technically pseudocanonical at best. While the answer itself is considered official, it too did not actually make use of the name either.

As for where the name originally came from, it is conceivable that some may have simply misremembered Optimus Primal's line in the final Beast Machines episode about "the great transformation" as having instead spoken of "the great reformatting", with the name subsequently having been spread around the fandom because it just sounded right.