This is based on the assumption, you're upgrading the QV Server only (not including Publisher)
First, prior to upgrade, I would suggest you make a backup of the original QlikTech folder. Depending on your operating systems, it's either:
-Windows 2008 (C:\Program Files\ProgramData\QlikTech); or
-Windows 2003 (C:\Documents and Settings\All Users\ProgramData\QlikTech)
Second, take notes on what settings have been done via Management Console: Tasks being scheduled, List of users assigned, etc.
Third, keep your License number & control handy. As far as I remember, I don't need to re-register the license, since it will automatically takes care itself. Depending on the outcome, you might want to have it. It's better to be prepared than sorry. I've done a couple migration from QV10 to QV11 with ease (thanks to the guidance provided by swuehl above . Since I've never encountered big issue, I don't need to revert thing back to the previous version.
If you do need to revert it, you might want just to re-run QV10 installation files (with the matching SR version you originally had or SR5 is the latest.
Note: You can find out the SR version by going to Console > [last tab] > System > About ). You'll see the version is shown as : Qlikview 10.XX.XXXXX. Refer to http://community.qlik.com/blogs/supportupdates to find relevant SR from the Qliktech download website.
This way, I believe it will keep the all the settings in Management Console intact as well. If you try to remove the QV11 from ControlPanel > Programs & Features, you might lose everything, as it will uninstall the whole Qlikview - not just QV11 (Disclaimer: I haven't done this - so you might want to check with others).
For me personally, I prefer to keep extra notes in case if something goes awry.
If everything goes smoothly, you don't need to do anything. All settings (task, the users, document, licenses) will work as it is original setup.
Do you know how many tasks or how many users that are currently stored/set in that client's server? If not many, you can just take notes easily. If it's alot of tasks, then writing them down may not be practical.
Note: Upgrading QV Server from 10 to 11is somewhat similar to upgrading firefox browser (if you're familiar with it). When you upgrade the browser, once it's done, the old settings (bookmarks, history) are kept intact. The reason is because they have similar infrastructure.
Upgrading QV9 to 11 or QV9 to 10 maybe slightly different though - again, refer back to the document that swuhel provides.