Gemara

12b (Beraisa - R. Meir): If the Kohen Gadol was temporarily disqualified and another was appointed, the first resumes serving (when he becomes fit again); all laws of a Kohen Gadol (permanently) apply to the second;

R. Yosi says, the first one resumes serving; the second cannot serve like a Kohen Gadol (with eight Begadim) nor (with four) like a regular Kohen.

R. Yosi: A case occurred, the Kohen Gadol became disqualified and another was appointed. Chachamim ruled that the first resumes serving, but the second cannot serve, not like a Kohen Gadol nor like a regular Kohen:

He cannot serve like a Kohen Gadol, for this will cause enmity;

He cannot serve like a regular Kohen, for we ascend in Kedushah, we do not descend.

Rabah bar bar Chanah: The Halachah follows R. Yosi. R. Yosi agrees that if he transgressed and served (like a Kohen Gadol), his Avodah is Kosher.

Objection: This is obvious!

Answer: One might have thought that we disqualify it lest he be like a rival in the Kohen Gadol's lifetime. Rabah teaches that this is not so.

Rishonim

Rambam (Hilchos Avodas Yom Kipur 1:3): If a Kohen substituted for the Kohen Gadol, afterwards the first resumes serving and the substitute steps down. All laws of a Kohen Gadol apply to the substitute, except that he may not serve as Kohen Gadol. If he served, his Avodah is Kosher.

Question (Kesef Mishneh): Since the Rambam rules like R. Yosi, why did he adopt essentially the words of R. Meir? Why did he omit saying that he cannot serve like a Hedyot?

Answer: R. Meir teaches that the latter is a Kohen Gadol regarding Hakravah and Isurim. R. Yosi essentially agrees, just he forbids to serve as Kohen Gadol because of enmity. Saying that all laws of a Kohen Gadol apply to him teaches that he cannot serve as a Hedyot.

Question: Mid'Rabanan, R. Yosi forbids the second to serve like a Kohen Gadol due to enmity. Since mid'Oraisa he is a Kohen Gadol, surely, he cannot serve like a Hedyot. Why does R. Yosi give another reason (Ein Yordim bi'Kdushah) why he cannot serve like a Hedyot?

Answer (Tosfos DH Kohen, and Ri Korkus): Even if the king or Kohanim will depose him to be a Hedyot again, he cannot serve like a Hedyot because Ein Yordim.