Can you upgrade the Logic Board of a 2018 MacBook Pro?

First, Apple never replaces any part with an upgraded part - if a repair is necessary, they will replace it with exactly the same. Second, unless the Mac is offered with custom ordering/upgrading different parts at time of original order, graphics cards cannot be upgraded later So, if your model was initially offered with an upgraded graphics chip as a custom order, you can ask an authorized service provider if they would do it and how much it would be. Applecare has nothing to do with any of that. If you have Applecare, best check first if such an upgrade would possibly void your warranty.

Apple does not sell parts nor swap/upgrade parts for any money. When a part is replaced, Apple insists the Authorized Service Provider return the old replaced part, and they have to match.

The only way to get your "dream" Mac is to convert the one you have to Cash (by selling it) and using the Cash to purchase the new or used computer you want to have.