Hebrews 7:9-12

Moffatt(i) 9 In fact, we might almost say that even Levi the receiver of tithes paid tithes through Abraham; 10 for he was still in the loins of his father when Melchizedek met him. 11 Further, if the Levitical priesthood had been the means of reaching perfection (for it was on the basis of that priesthood that the Law was enacted for the People), why was it still necessary for another sort of priest to emerge with the rank of Melchizedek, instead of simply with the rank of Aaron 12 (for when the priesthood is changed, a change of law necessarily follows)?