I've already preordered my copy of the Mists of Pandaria Collector's edition, the physical version from Amazon, but I don't want to wait until the mail comes to play. What happens if I buy a digital version, then redeem a physical edition? Do I get reimbursed for any of the digital purchase?
No. Once you add a game key to your Battle.net account, it is attached permanently. It cannot be removed or transferred, and Blizzard has never offered refunds, to the best of my knowledge, for "redundant" Standard Edition keys if you later attach a Collector's Edition key to your account.
The only exception to this that I'm aware of is World of Warcraft Annual Pass customers getting four free months of game time if they purchased the Diablo III collectors' edition, but that was a fairly specialized case.
That said, Amazon usually offers release date delivery for games. If it's available to you, upgrading to release date delivery may be a cheaper option that will still allow you to play when the game goes live.