I think the only thing you might loose is polls and Sticky topics.
Meaning they will revert to a regular post.
I don't think you had polls in your version but if you did. What you would loose is the results.
I went from classic 6 to threads 7.2 with very few issues 2 years ago. Other than apprehension if it did not work and the completely new interface in the control panel.
One thing I did do while still is classic is a lot of cleanup of forums, posts and users. And also to make sure I did not have any hacks or mods still lingering.
I did have some problems with the redirect scripts working correctly but it was something I was not to concerned about.
