Well you are going to need to use DiRT to maintain the install. Keep in mind that DiRT requires you to restore to the same version that you did the backup from. So you will either need to upgrade 3.1(3) to 3.1(5) backup, reinstall with 3.1(5) and then restore or backup 3.1(3), reinstall with 3.1(3) restore from DiRT and then upgrade to 3.1(5). Either will get the job done.
Also, depending on how your environment is setup you may or may not want to backup the current voicemail messages. DiRT has the option to do so. What does your environment look like? If your email store if off box you don't need to back the messages up. If it is on box then you will.
Always make sure to get the latest copy of DiRT from:
http://www.ciscounitytools.com/App_DisasterRecoveryTools.htm
Hope this helps,
Keith
