A:Answer I don't believe you do, but Blizzard said in their post of it becoming available that if you preorder the digital deluxe, you will immediately get the rewards, and once the physical collector's edition becomes available, you can input the key for that onto your account, and they will give you credit for the digital pre purchase as Battle.net balance.
Original post for reference:
"If you’ve purchased a digital copy of Battle for Azeroth from our in-game shop or Blizzard shop, 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."
