I recently made this upgrade, and it seems to be working fine. As far as I'm aware, all the system-wide files sit under the one directory (/usr/local/OpenOffice.org1.1.0 or whatever) so you can just obliterate that. That's what the OpenOffice install guide says too.
In terms of per-user installation, you need to be aware of the ~/.sversionrc file. This points to where the per-user data is to go. The instructions say to delete these, which will mean that when the user goes to run OpenOffice it will go to the reinstall process to set up the new version. Obviously, you then lose all your settings. Maybe try just changing the version number in .sversionrc? Worst thing that can happen is it doesn't work, and you delete the file anyway!