Ramban on Shemot 28:1 notes that only Aaron and his four sons become Kohanim upon the inauguration of the mishkan, and that Aaron's grandson Pinchas is excluded from the priesthood:
AND THOU BRING THOU NEAR AARON THY BROTHER, AND HIS SONS WITH HIM … THAT THEY MINISTER UNTO ME IN THE PRIEST’S OFFICE, EVEN AARON, NADAB AND ABIHU, ELEAZAR AND ITHAMAR, AARON’S SONS. The reason for mentioning Nadab and Abihu, Eleazar and Ithamar [when it mentioned already “and his sons”] is that Moses should not think that by anointing the father to minister as priest, his sons would automatically become priests; instead he had to initiate them personally into the priesthood. Thus Phinehas [the son of Eleazar] and others already born were excluded [from the priesthood], for only these four sons who were anointed with Aaron, and their children born to them henceforth, were appointed as priests.
Pinchas doesn't become a Kohen until he is rewarded for slaying Zimri (Rashi on Bamidbar 25:13)
A COVENANT OF AN EVERLASTING PRIESTHOOD — for although the priesthood had already been given to Aaron’s descendants (cf. Exodus 28:40-41), it was given only to Aaron and his sons who had been anointed together with him and to their offspring whom they might beget after they had been anointed. But Phineas who had been born prior to that and had not been anointed, had not as yet attained the status of priesthood until now. So, too, do we read in Zevachim 101b: Phineas did not become a priest until he had slain Zimri.
Why wasn't Pinchas (or any other living grandsons) included in the inauguration with Aaron and his sons? Why should it matter that he was already born, in contrast to the other grandsons of Aaron who automatically inherited the priesthood?
