After the One Year War, the Musai type was modernized and modified to become the main ship owned by the Republic of Zeon. The Musai Kai-class's basic structure is not much different from its predecessor, but it has two new secondary guns, and other modifications were made to the thermonuclear reactor and the radiating mechanism. Fins were added to the engine blocks and the sides of the main body, and a pair of antennae were added to the bridge tower. It is unknown how many mobile suits they could carry, but it can easily be said that they were capable of carrying at least five of the new RMS-106 Hizack mobile suits like its predecessor: three in the hangar and two in its Komusai shuttle.
After the One Year War, the government of the Principality of Zeon was restructured back into the Republic of Zeon, an independent state that was allowed to maintain its own military force as long as they abide by rules and regulations set down by the Earth Federation and comply with regular and random inspections. The Republic of Zeon designed and used a new upgraded version of the Musai as their light cruiser - the Musai Kai-class.
By U.C. 0087, most of the surviving Musai-class ships had been upgraded into the Musai Kai-class by the Zeon forces at the asteroid Axis. This subclass was used by the Republic during the Gryps War and well into the U.C. 0090s. During the Gryps War, the Republic of Zeon would supply the Titans with several Musai Kai-class ships. The Titans would use these ships during Operation Apollo and the evacuation of the Gate of Zedan.
At least one Musai Kai-class was used by the Neo Zeon remnants "Sleeves" in U.C. 0096, and was stationed at the Palau mining asteroid colony. It was destroyed when the Nahel Argama used its Hyper Mega Particle Cannon to destroy Palau's military port.