From the intuit website:
Q: Will my data from previous QuickBooks versions be transferred safely if I upgrade to QuickBooks 2009?
A: Yes, when you open QuickBooks 2009 the first time, the new software will find the older version of QuickBooks on your computer, and import your company file and preferences for you.
Q: Can I upgrade all older QuickBooks software versions to the latest version?
A: Yes. If you're a Windows user, you can upgrade from all older versions of QuickBooks to any version of QuickBooks 2009 for Windows. You can also convert data from some QuickBooks Mac editions to QuickBooks Windows editions. (Additional software may be required.)