I would never do this mid-project, but I need to upgrade to 6 from 5 because my deliverable has changed to ProRes. I know there has been some drama in the past about this, but so far I have only one project in progress, which is all P2 DVCPro HD imports. I have done no effects or color correction or speed changes. Just a few cuts-only sequences from interviews. There is probably 3-4 hours of footage in there now.
Can I get away with this? I'll save a 5.14 version my project first...
if so then I'll only be one version behind on FCP!
PS can't use version 7- no MacPro.
you could do a clean install of system and FCP 6 on another internal drive, or FW drive. This way you can easily see if it works to open the project file of your FCP 5 into FCP 6. If not, try an XML export and import that into FCP 6. And you'll still have your old system and FCP 5 as a backup..
Search this forum about how to do an upgrade. It has been covered here many times and the resounding advice is a clean install of OS and FCS on a new drive, keeping the old drive as an alt boot so you can boot either.