Thank you soontobe for clarifying why you think God didn't plan when he created the world and later changed the rules. (I am not sure why you suggest that I would think Hebrews is an Old Testament book of the Bible but I managed to find the passage you refer to without trouble and, in fact, re-read the whole book as it is short.)
Surely the letter is about how Jesus supersedes all the previous High Priests because he is the eternal Son of God and the eternal High Priest. The mere human priests of the Old Testament and their merely human rituals and sacrifices could only foreshadow true salvation and Jesus represents the fulfilment of God's promises to Israel.
The unknown writer also makes it clear that Jesus is the superior of angels – reassuring for some Christian communities of the time and, presumably this one (also unknown) – because they were believed to presage the end of the world.
He also makes a big point about Jesus being superior to Melchizedek, the eternal High Priest of the Old Testament. This is clearly important to the troubled Christian community to whom the letter is addressed because this superiority to Melchizedek confirms Jesus's superiority to the Levitical priesthood.
The letter's purpose seems to be to encourage the faith in a wavering community. It declares that Jesus is God's final revelation, completely the opposite of a change of rules and a change of plan.
This is not really the place for such a discussion and it will bore the pants off other Gransnetters. I shall, therefore, shut up now! [resounding cheers]