In 357, Fu Jiān overthrew the cruel and whimsical Fu Sheng (who had, in addition to many others, put Fu Huangmei to death even after his great victory) and took the throne himself. Sometime early in his reign, Fu Jiān made Yao Chang a general. The first campaign in which he was mentioned by name was in 366, when he assisted
Wang Meng in an attack on Jin's Jing Province (荊州, modern
Hunan and central and southern
Hubei). In 367, he again assisted Wang Meng in attacking the rebels in modern eastern
Gansu, who were led by the Qiang general Lian Qi (斂岐), whose subordinates were originally all Yao Yizhong's subordinates and therefore surrendered to him readily. Fu Jiān made him the governor of Longdong Commandery (隴東, roughly modern
Baoji,
Shaanxi). In 371, he was part of the campaign against
Chouchi's ruler
Yang Cuan (楊篡), and in 373, after Former Qin seized the modern
Sichuan,
Chongqing, and southern
Shaanxi from Jin, Yao Chang was made the governor of Ning Province (寧州, modern southern Sichuan). In 376, Yao Chang also assisted Gou Chang (苟萇) in Former Qin's conquest of
Former Liang, and in 378 involved in the siege against the Jin city Xiangyang (襄陽, in modern
Xiangfan,
Hubei). Sometime during Fu Jiān's reign, he created Yao Chang the Marquess of Yidu. In 383, Fu Jiān prepared a major attack on Jin, intending to destroy it and unite China. Yao Chang was one of the few generals who was in favor of the plan, as the vast majority of Fu Jiān's subordinates opposed, including his brother and prime minister Fu Rong—who specifically was suspicious of Yao Chang and
Murong Chui (a general with
Former Yan imperial ancestry), neither of whom was of Di ancestry and both of whom favored the attack on Jin. Fu Jiān proceeded with the plan despite Fu Rong's opposition, making Fu Rong the commander of the main invasion force. Yao Chang was put in charge of the armies of the southwest (on a separate front away from the main front), and as Fu Jiān was handing out the command assignments, he gave Yao Chang the title General Longxiang (龍驤將軍), a title that he himself carried previously when he overthrew Fu Sheng, and he commented, apparently in a jocular tone: :
Before, I established my rule as the General Longxiang. I do not easily confer this title on others. You should take good care of the title. Another general,
Dou Chong, immediately objected, stating that it is a bad omen for the emperor to joke, and Fu Jiān grew briefly silent. Later that year, against the much weaker Jin troops, Fu Rong's forces were nevertheless defeated at the
Battle of Fei River, a battle that Yao Chang was not involved in, and Fu Rong was killed. In early 384, Murong Chui rebelled in the eastern empire, hoping to reestablish Yan, and upon hearing Murong Chui's rebellion, Murong Chui's nephew
Murong Hong rebelled as well, near the Former Qin capital
Chang'an. Fu Jiān sent his brother Fu Rui (苻叡) against Murong Hong, assisted by Yao Chang. Yao Chang advocated letting Murong Hong, who was intent on leaving the
Guanzhong region to return to his homeland, leave, but the brash Fu Rui insisted on intercepting Murong Hong, and was defeated and killed by Murong Hong. Yao Chang sent messengers Zhao Du (趙都) and Jiang Xie (姜協) to report the bad news to Fu Jiān and Fu Jiān, inexplicably, executed Zhao and Jiang in anger. Yao Chang became fearful and abandoned his troops, gathered the Qiang of the region, and rebelled himself, claiming the title of "King of Qin of 10,000 Years" (萬年秦王), thus establishing Later Qin. == As king ==