The deluxe upgrade will likely be available post launch.

Currently there doesn't seem to be a way.

Emphasis on "currently". If Rise and World are anything to go off you'll be able to do so after launch. The Deluxe editions for both of those games are sold as DLC packs, allowing you to upgrade the standard edition if you wish.

Alternatively contact Steam Support and see what they're willing to do. I don't know offhand if it's possible to refund a gift into your Steam wallet or if a gift can only be refunded "to" the person who bought it.

theres no extra preorder bonus for the deluxe version. you will be able to upgrade to the deluxe version after launch. itll cost the difference of the base price and deluxe price.

keep in mind, deluxe version is purely cosmetic items with no additional gameplay content added.

the cosmetic packs that come at a later date is likely the spring seasonal event set and summer summer event set but thats just my speculation.