If you are like me and pre-ordered the digital release of BfA to get the early perks, but still want the box version of the Collector's Edition, here is Blizzard's official word of how they will deal with refunds:
"Upgrading from Standard or Digital Deluxe to Collector’s Edition
If you’ve purchased a digital copy of Battle for Azeroth, but want to upgrade to the Collector’s Edition, we will automatically credit you the cost of the digital copy in Blizzard Balance - as long as you enter the Collector’s Edition key before 11:59pm PST on December 31, 2018. When you enter the key from your Collector’s Edition, we will see the original transaction on your account and the credit will kick in. Remember, you will need to enter the Collector’s Edition key before the end of the year, as automatic Blizzard Balance credits will be turned off at 11:59pm PST on December 31, 2018.
Please note, if you bought a standard edition then upgraded to a digital deluxe, you will be credited for the original copy only. "