This option is a safety measure to prevent an update from swapping views if a crawl gathered too few documents.
At the end of an update the live and offline collection views are swapped to make the new documents, and indexes, available to users. Before the swap is performed, the number of documents in the older live view is compared to the number in the newer offline view:
If the percentage:
(number of new documents / number of old documents) * 100
is less than the
changeover_percent value then the swap will not proceed.
By default the new crawl only goes live if the ratio of new vs. old documents gathered is greater than 50%.