Assume everything can change.
Assume anything could still change, but limited change should be expected.
Change is extremely unlikely. A high threshold will be used, and a change will only occur because of some truly critical issue being identified during the public review cycle. Any other desired or needed changes can be the subject of a follow-on new extension.
No changes are allowed. Any desired or needed changes can be the subject of a follow-on new extension. Ratified extensions are never revised