Have you tried running the online update from behind your firewall and choosing one of the http mirror sites? Only the main suse site uses FTP, the others are http (the downloaded files get "wrapped" in http headers so as far as your corporate firewall sees, they are web pages). For example, I use the site http://ftp.uni-bremen.de/pub/mirrors...suse.com/suse/
as my update source, although I'm not behind a corporate firewall so I can't say for sure it will work for you!
When you are "changing the source of update", I think all you are doing are changing the location to look for additional
packages to install or reinstall, (i.e. the place that is looked at when you click on "Install and remove software"). This is not the same as the location for updates.
Hope this is useful,
Freelance Lecturer in Computer Science