I would agree with topher145...however as a precaution, I would make a copy of the notes.ini file and the data directory somewhere. If you have to go back for whatever reason, simply re-install the previous version of Domino and replace the installed versions of the notes.ini and data directory with the copied files.
Similarly, with BES upgrade (there isn't a downgrade path) copy the BES\State directory and the profiles database (there are some others) before you proceed. There are significant changes made to the domino files which become integrated with the SQL database.