Must be greater in wisdom and in number to override a previous Beit Dins ruling
A Siyag cannot be overridden by another Beit Din even if they're greater in wisdom and number unless the Siyag did not catch on.
Before a Beit Din wants to enact anything they must check to make sure the community can uphold it.
If the Beit Din went ahead and enacted a gezera (under the impression that most Jews uphold it) and it was not upheld by most Jews, then the gezera is null.
If the Beit Din was under the impression for many generations that the gezera spread throughout most Jews and a later Beit Din discovered that it never spread throughout most Jews, they can nullify the gezera etc. even it is not greater than the Beit Din that enacted it.