If you have no qualms about completely losing your configuration and having to start over from scratch, I'd categorize it in the "EXTREMELY low risk of losing data" department. I have one or two potential caveats to that, though.
A fresh FreeNAS install has a reasonably decent ability to import existing filesystems, especially ones previously created by FreeNAS. What that means is that you can rip out your old FreeNAS 8.0 flash and stick in your new FreeNAS 8.2 flash, import, and then spend a few hours recreating your config if you didn't happen to save the old one.
But when you do, you should be up and running.
Configurations are NOT guaranteed to be importable between versions except for ... released code, I think? We ran into a situation where someone had suggested that we run a nightly build to see if a problem went away, but basically it resulted in a config db that was incompatible with future versions, and upon trying to import it, it turned out that not only was the code non-resilient to rejecting an incompatible configuration, but it actively rendered the install unfixable (short of going in and manually editing the database). I believe that they make reasonable attempts to ensure that BETA candidates can be upgraded to RELEASE versions, but beware and be prepared. In particular, don't invest incredible amounts of time in a thorough FreeNAS configuration that sets every possible bit in every menu, etc.
I would also advise against trying to import a broken pool of some sort, such as a pool with a failed or missing disk. While I'd wager a small sum that it would manage the situation successfully, the fact of the matter is that it's uncommon, and uncommon things are less tested.
