Like verse 7, this is a statement of one of the principles on which the argument is based. Since the Law is based on the priesthood (verse 11), a change in the priesthood entails a change of law. There is no the before law in the Greek text. The author means that one entire legal system is replaced by another state, namely the new covenant, not that one or two of the Old Testament laws are changed. In Hebrews the main interest in the Old Testament Law is in its regulations for worship. Has to suggests logical necessity, not any outward pressure.

If the statement in verse 12 is to make sense, priesthood must be understood as “the system of priests” or “the kind of priests.” Therefore when the priesthood is changed may be rendered as “when there is a change in the kind of priests,” “when there are no longer the same kind of priests,” or “when the kind of priests is different.” The manner in which one renders the clause there also has to be a change in the law depends on the way in which the first part of verse 12 is translated.

Quoted with permission from Ellingworth, Paul and Nida, Eugene A. A Handbook on The Letter of the Hebrews. (UBS Handbook Series). New York: UBS, 1983. For this and other handbooks for translators see here .