Like his fellow Gundam pilots, Chang Wufei returns to space with his Shenlong Gundam to counter OZ's appeal to the colonies. When the XXXG-01S Shenlong Gundam was captured and destroyed by OZ's forces in AC 195, the extremely durable gundanium-made remains of that mobile suit were taken to OZ's lunar base and mobile suit factory for storage. Wufei allows himself to be captured, and the five scientists who created the Gundams proceed to secretly upgrade his Gundam at OZ's lunar base.

The improvements given to the new XXXG-01S2 Altron Gundam were very dramatic. The old beam glaive was replaced with a double-ended twin beam trident, and both arms were outfitted with the highly effective self-contained dragon fang claws (hence the Altron name, taken from the Chinese "ertou long" or "two-headed dragon"). Lastly, a set of "random binders" on its backpack give the Altron better space mobility. Along with increased mobility, these upgrades turned the resurrected "Nataku" into an incredible force, allowing Wufei to continue his fight for justice against oppression.

Source: Gundamproject.com, Gundam W: Endless Waltz DVD