Very pedantically, because PHP doesn't require copyright assignment, it would be (almost certainly) impossible to retroactively change the licence on older versions.

However, since the PHP and Zend licences both permit the user to use PHP under the terms of whatever licence version was applied to that PHP version or any later version, the point is essentially moot, since a user can choose to use the new version of the PHP/Zend licence once published, which will give them the same rights.