OK I have had to do this recently and often enough to have a plot!
I would strongly recommend backing up your data and reinstalling from scratch. It has proven for me to be much easier and quicker in the long term and makes this easier next time
When I have done it I also have to plan in advance (usual back of envelope stuff!)
1 I tend to back up my home personal folders on an external hard drive (things like music/pictures etc). I usually wipe the whole drive but you may prefer if you have no external usb to create a new partition.I am not skilled in this.
2 I check the backups work...check a few files and folders etc to prevent tears
3 I then also back up the following
firefox bookmarks (as a .json file)
evolution settings (export settings)
desktop backgrounds (depends where you save them but usually in usr/share)
I then note all the applications I have installed and or need to reinstall. For me it is not many - Skype/Dropbox/Wifi Drivers/Samba/vlc etc and check the work in 11.04
4 I then (like you have) check the live usb/cd rom works!
5 If it does I install and wipe the whole drive following the prompts on the install procedure.
When installation is complete I reboot and do the following:
6 Make sure all the repos are ticked and enabled in synaptic package manager eg third party backports etc
7 I then install all the updates and applications I need with one big terminal command which I can leave running which will update the pc since the release of Ubuntu 11.04.
For instance in the terminal last time I ran this:
sudo apt-get update && sudo apt-get dist-upgrade
I then reboot and run this
sudo apt-get install ubuntu-restricted-extras && sudo apt-get install skype && sudo apt-get install samba && sudo apt-get install vlc && sudo apt-get install bcmwl-kernel-source && sudo apt-get autoclean && sudo apt-get autoremove
Feel free to remove any of the above that you do not want as it was just what I needed.
This has set me up several times now and except for clicking ok for some of the restricted extras there is nothing much to do.
8 Install dropbox from their website .deb and that backs up work stuff for me
9 Import both the firefox bookmarks .json and evolution back up and save them locally in my Downloads folder.
10 I set up evolution from the menu at the top indicator using the back up file which saved me alot of messing around and imports my mail too.
11 I then set up broadcast and chat accounts
12 Import back the backup folders - music pictures etc and get some music back on
13 Enjoy what you will find is a red hot new Ubuntu :)
14 Have a look at other what to do now you have set up 11.04 guides like the excellent omgubuntu site here