So years ago (2007) I created a development server using Ubuntu Feisty Fawn. At the time i thought nothing of it until recently when I couldn’t use aptitude to install stuff and after doing some research found out that Feisty Fawn is no longer supported. So right off the bat it is recommended that you deploy LTS (long term support) versions of ubuntu on commercial servers.  I got hit with a double whammy (the next available LTS version 7.10 was also outdated) so I had to upgrade to 8.04 which despite info on the ubuntu website wasn’t straight forward because of bugs in the different installers. This is really the same info found on the ubuntu website

1. You have to upgrade to 7.10 then from there upgrade to 8.04 so download the alternate version of Gutsy release (http://old-releases.ubuntu.com/releases/gutsy/) and burn to a cd because network install is no longer available for this release.
2. Next you need to ensure you have the latest feisty fawn stuff installed. Comment out the repos in your current /etc/apt/sources.list and adddeb http://old-releases.ubuntu.com/ubuntu/ feisty main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ feisty-updates main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ feisty-security main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ feisty-backports main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ feisty-backports main/debian-installer
deb-src http://old-releases.ubuntu.com/ubuntu/ feisty-backports main restricted universe multiverse

   And then run sudo apt-get update && sudo apt-get dist-upgrade

3. Comment out all the lines in /etc/apt/sources.list again and now we’re going to add the cd that we created earlier to that sources.list. To do this do sudo apt-cdrom add /media/cdrom0 and follow the instructions (they basically say insert the cd and it does it’s thing)
4. Now do sudo apt-get dist-upgrade and the upgrade should work. Once you’ve managed to upgrade to 7.10 (now defunct LTS version) you can make you way to the most recent LTS version at the time of this writing (8.04) by doing another sudo do-release-upgrade

You may/may not have php problems (if you had that installed) because the suhosin patch is installed by default in this version. See this post for help with getting a non-suhosin version of php installed.