I'm about to uninstall the D3 public beta and install the pre-release download. Is there any overlap in files (that I'd be deleting and re-downloading)? If so, what do I need to do? I'd prefer to preserve the bandwidth, if possible.
AS @Krazer said in the comment of your question.
And Bashiok in his Twitter.
And in this answer I say all files that are on MAC. And there is a file that saves your preferences called D3Prefs.txt the location is: /Application Support/Blizzard/Diablo III
But as @Bashiok said in his Twitter, the beta is different from the retail version, besides, all your champions and preferences are gonna be reset. So you don't need save any files. And if you did, they are useless to the retail version.
(You asked the question of the answer that I linked above, so I'm assuming you could use this path better than a windows one. But you can find those files on windows too (some of them at least))