Hi pgplast,
Before you upgrade, make a backup of the entire old database and then the entire old cmsAdmin folder. (make sure you can tell which backups are the latest, because dates get overwritten when files are transferred.)
That way you always have all files in their original form.
You may have to upload some of the new schema files to see the new fields in the sections, but again, if any issues arise you always have the old backup available.
Remember, its a bit more work, but you can always do a completely new parallel install in a new folder on your server and not delete the old installation until you've squashed any errors and you're happy with the result.
There's some more detailed information in my CMSB Cookbook in the recipes MOVE A CMSB SITE FROM ONE SERVER TO ANOTHER WITHOUT FEAR and IMAGES WON’T SHOW AFTER MIGRATING TO A NEW SERVER - THE FIX IS IN
Hope that helps,
Jerry Kornbluth
The first CMS Builder reference book is now available on-line!
Take advantage of a free 3 month trial subscription, only for CMSB users, at: http://www.thecmsbcookbook.com/trial.php