What the upgrade does is overwrite all files except the app/local and the default theme. If you had a backup of all the magento files prior to the upgrade you could downgrade back to 1.1.6 and restore your 1.1.6 database backup.
The best practice that I have found is have a development website and a production website that are pretty much identical. That way when you do an upgrade do it on the develpment website and see what happens. If it gets all messed up you could simply copy over the develpment with the production and start all over.
The process is similiar to moving your site to a new server as shown here.
Hope this helps.