When Moshe is reluctant to accept the responsibilities of leadership, refusing God’s repeated requests that he return to Egypt as leader of the Children of Israel (see Sefer Shemot Chapters 3 and 4), God ultimately becomes angry with Moshe and tells him that his brother Aharon the Levite will speak on his behalf (Shemot 4:14). Rabbi Shimon bar Yohai interprets this passage to mean that God had originally planned to make Moshe the priest and leave Aharon as a levi. Since Moshe refused to accept responsibility as a leader, God was going to switch their positions so that Aharon became the kohenand Moshe remained a levi. The Hakhamim teach that Moshe did serve as a priest for a limited amount of time – during the seven days of the consecration of the Tabernacle (see Sefer Vayikra Chapter 8) – when Moshe performed the sacrificial service. Yet another opinion is quoted in the baraita, that Moshe remained a kohen his entire life and it was only his children who did not merit to remain kohanim, as opposed to Aharon’s children, who retained their family status for all generations. In support of this last position, a passage from Sefer Divrei ha-Yamim (I 23:14) is quoted that says that Moshe’s children were associated as Levites; furthermore, the passage in Sefer Tehillim (99:6) refers to Moshe and Aharon together as kohanim.
Rav teaches that Moshe, in fact, served as the High Priest, as we see (Vayikra 8:29) that he received the High Priest’s portion during the days of consecration. It appears that according to this opinion, Moshe retained the position of High Priest throughout his life. In his Keren Orah, Rabbi Yitzhak of Karlin explains that it is logical to assume that once Moshe attained this position that he did not relinquish it, although on a practical level, once Aharon was anointed as kohen gadol, Moshe allowed him to serve in that position, as he was occupied with matters of leadership and direct interaction with God.