hey man if u still need info about ur question I did it yesterday - contacted steam support and they sent me exact procedure how to do it - here is there original msg - so u can give it a shot, hope it help u
Hello,
I understand you want to purchase the Deluxe Edition instead of the Standard one. I'll try to help you with that. Here is what you need to do:
First, remove the game Destiny 2: Shadowkeep from your account by following these instructions:
Log in to your Steam account here: https://help.steampowered.com/
Use the Search field to find any games you want to remove.
Click on "I want to permanently remove this game from my account"
Then go ahead and purchase Destiny 2: Shadowkeep Digital Deluxe Edition. Once the Deluxe Edition is purchased, please write back to us, confirming that and we'll refund your money for the purchase of Destiny 2: Shadowkeep. Please specify if you want those funds back to your card or your Steam wallet.
I did it and saved even 25 % bc of discount - so for like 500 silver difference I got emblem, ghost and emote but most important +3 upcoming seasons( 3000 silver total)