I'm a bit confused here, so is his actual identity Zhao Sheng? Or is it another name he stole? Because he does say "Not only will I replace Zhao Sheng" on page 30
Zhao Sheng is a fictional character in the Zhang school of Daoism. He is modeled after the "brilliant disciple" archetype, a seeker of knowledge who himself is already more than prepared to receive the master's instruction. He is only there to become the test-taker for the Seven Trials of Zhao Sheng story (like the seven signs/miracles of the Gospel of John) passed down in the sect's mythology to show the greatness of the founder.
In this story, "Zhao Sheng" is merely a name passed down from one celestial master of the school to the next to create the illusion that they were immortals. The Eighth is the current generation's Zhao Sheng. He is replacing the previous Zhao Sheng.
Interestingly, the practice described by Yuan Fang is historically attested in at least one case. The title "Master Changchun" was passed down (or serially adopted) by several Daoist masters over a 200-year period, culminating in Qiu Zhuji, the founder of the Dragon Gate sect. Qiu was the one who basically revealed the trick to the world. When Qiu was summoned by Genghis Khan and pressed to share the secret of immortality to the Khan, he openly admitted that there was no such secret, that he himself wasn't immortal, and that legends of him being over 200 years old came about because his title was passed down from master to student.