Charles Spurgeon Commentary Hebrews 7:20-24

"And inasmuch as [it is] not without the taking of an oath (for they indeed have been made priests without an oath; but he with an oath by him that saith of him, The Lord sware and will not repent himself, Thou art a priest for ever); by so much also hath Jesus become the surety of a better covenant. And they indeed have been made priests many in number, because that by death they are hindered from continuing: but he, because he abideth for ever, hath his priesthood unchangeable." — Hebrews 7:20-24 (ASV)

I think they calculated that there were eighty-three high priests in regular succession from Aaron to the death of Phineas, the last high priest at the siege of Jerusalem. One succeeded another, but this one continues perpetually, forever has an untransferable priesthood.

The word 'untransferable' is closer to the meaning than 'unchangeable.' If any of you have older Bibles with marginal notes, you will see it says, 'has a priesthood which cannot be passed from one hand to another,' and the marginal note, in this case, happens to have the true rendering: 'This man has an untransferable priesthood.'