In order for Jesus to be declared High Priest by God, there had to be a change of priesthood so there could be a change of law. Why? Because according to the law given by Moses to the Jews, priests were required to come from the tribe of the Levites. Jesus was from a different tribe (Judah) which the law said nothing about concerning priests.
The law was received by the Jews through the (Levitical) priesthood. They fit together like a hand in a glove, and you could not eliminate one without doing the same with the other. Those many priests ministered within a covenant that was weak and useless ... and would be replaced by a superior priesthood—Jesus—who was not appointed by the law like the other priests, but would carry on an eternal priesthood. If the Mosaic law still had any function whatsoever, then Jesus could not be considered as a legitimate priest. It had to end.
In Hebrews 7, the writer is explaining why that law had to completely be annulled and cancelled—not revised or tweaked.
