Installing and upgrading help

I'm trying to modify the functionality of the existing auth_email plugin (email based self registration) The only change in functionality is to send an additional email to users who are based in a certain area based off their IP Addresses. We made these modifications and then when tested it on our local MAMP server it passes the checks. But when we upgrade the database it breaks the site administration completely. Could something be wrong with the settings.php? Does anyone know what could have caused this? Also not happy with this approach, it would be better if we could build on top of the plugin, does anyone know a better route to take?