For RC1, it'd be best to do a clean install according to my conversations with Thrax. Best to give it a fresh start, right? We'll probably find out more next week.
There's actually a check in place that will attempt to prevent you from upgrading from a beta copy to RC1. It can be gotten around, but why tempt fate? Go clean to give it the best chance to continue wowing you!
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How is the upgrade process? Do you just have to do a clean install, or can you upgrade from your prior Beta version?