I actually made the mistake of installing only WSS when I actually wanted the MOSS version. I had completed all the configuration and created some test sites.
To perform the upgrade, I simply ran the MOSS installation and it updated the server with the extra components without losing my sites. The only thing I had to do was select the option in the administration screen to enable features on all existing sites to add the new web parts and workflows.
If you are going to create your own sites and features, then I would take a backup of these first and check they are enabled after the upgade.