Consider this example:
<?php
$zip = new ZipArchive;
$zip->open(...);
$zip->addFile('path/file', 'foo');
$zip->renameIndex(0, 'bar');
echo $zip->getNameIndex(0); // 'bar'
$zip->unchangeIndex(0);
echo $zip->getNameIndex(0); // 'false'
?>
Unless you call save() in between, the unchangeIndex() call reverts back to the initial state of the archive - where index '0' did not exist.
If you called save() after addFile() and then renamed the file, you would be able to revert/undo the name change.