(1) On application under section 17, the commission must—
(a) amend the approval; or
(b) refuse to amend the approval.
(2) In deciding whether to amend the approval, the commission must have regard to the matters mentioned in section 12 (2) and (3) (Decision on application) as if the application were an application for an approval.
(3) An amendment of an approval has effect only if the amendment is stated in the approval.
(4) To remove any doubt, the amendment of an approval under this section may include the imposition of a condition on the approval and the amendment of a condition to which the approval is subject.