Translate in the Greek order, “For they indeed (the existing legal priests) without the (solemn) promise on oath (so the Greek [Tittmann]) are made priests.”
by him - God.
unto him - the Lord, the Son of God (Psa 110:1).
not repent - never change His purpose.
after the order of Melchisedec - omitted in some oldest manuscripts, contained in others.