Dear Judy,
I think that given your description that it should not be an issue. For the existing users they will not see the change until their page is refreshed in some way like clicking on a link, AJAX request.... This is as their view (client end) would have been created as a result of the processes and data available (server end) at the time of the original request.
I assume you have a backup of the original 'lib.php' file if things go wrong.
Cheers,
Gareth