This is my experience when I replaced my old Gutsy with Intrepid.
When I use Linux, I always divide my hard disc into 3 partitions, System, Swap and /Home. It’s a good idea because next time you want to replace your OS, you will not lost all your /Home data.
For your Virtual Box, you will still have most all of your old configuration and the image file. When I did this upgrade, I still have my old virtual box setup and my OS image inside it.
I said not all because the ‘saved state‘ you can’t get it back. You have to press the ‘Discard‘ before you can use it. But it’s ok, only take few seconds for an OS to reboot.
Below are the steps.
First, go the virtual box website and download the program.