If you enter the key, and it complains, then you know it's time to re-issue it. Generally if you're doing a new install and it's not a fresh key, when it asks you for the key, that's the time to do the re-issue.
I suggest doing the new install and selecting the 30 day free trial, because then you'll have both a trial, and your official keys allowing you to run 2 copies of v3 side by side.
Once your new install is done and migrated and ready to go live, re-issue your official key and plug it in under Settings > System > General > License Key