You should upgrade to the latest SVN with the help of the How To: SVN document in the "Vicidial Wiki" at the top of this page.
Or just reinstall with the .iso image from Vicibox.org (for a nice clean install ...). If you build a new one, then export your data and import it into the new one ... you will experience NO downtime and no risk (ie: you can always just go back to using the old one any time you like!).
If you upgrade your existing server, you can get the new code via SVN and read the included UPGRADE document before any changes are made to your system. Once you begin the upgrade, however, your system will be down until you complete it. We've never had a failed upgrade ... but since that is nowhere near a guarantee, I highly advise you to get a good backup and verify that the database will load on a 2nd server to be sure your data is safely stored off the system before you begin the upgrade. If you do not load the DB onto another mysql instance ... you'll not be guaranteed to have your data safe. Not a good feeling.