You can virtualise it but I am not familiar with the tools. I use VirtualBox on my PC but it is only a Mickey Mouse set up. I think when I went from 5 to 6 I set up 6 in VirtualBox to sync all the data and e-mails out then wiped the old server and did a fresh install of 6 and sync'd all the data back. For 6 to 7 I set up an old PC temporarily and installed 7 on it, dis a 6.x backup -> 7.x config restore and sync'd the data across. Then I moved the disk from there back into the production box. It leaves a few issues with interfaces which change between boxes but was not too difficult to sort.

Thanks Nick. I need to back up and re-install the data I'm not looking forward to this at all. I may virtualise the old server to allow me to continue to use it while Build the new server- is there and easy way to do this in Linux? P2V for Linux?

Sorry but it is a full re-installation. It is possible to do a 6.9 config backup and restore into 7.x for your users and configs but you then need to pull all your data and e-mails across. rsync is a good tool for data, and, if you know the passwords, imapsync for e-mails.