plugin modification

First of all I really like this plugin and it seems to be the only one that is not completely uncustomisable.
I’ve ended up modifying it quite a bit to meet the needs of my project and want to make sure nothing is overwritten during an update. I renamed it, of course, but was wondering about this : require_once(ABSPATH . 'wp-admin/includes/upgrade.php'); I thought I might just remove it but I don’t want to break anything either. Please advise.

http://ww.wp.xz.cn/extend/plugins/simple-event-attendance/