Any change to a Cochrane review is either an update or an amendment.
An update must involve a search for new studies. If any new studies are found, these must be added to the relevant section of the review as included, excluded or ongoing studies (or ‘Studies awaiting classification’ if all reasonable efforts to classify it one of these ways have failed), before labelling the revised review as an update (see Section 3.2.5.1).
Any other change to a Cochrane review, and any change to a protocol, is an amendment, which could involve a little or a lot of work. These terms, and when to apply them, are described in more detail in Section 3.2.3.