So essentially, would you get a free upgrade, a whole new upgraded card or the money for it? Choosing one new upgraded card from all of the options could be confusing. I support the idea but the implementation, I think, is part of the discussion.
I'd make that the reward for beating your first halfblood.Halfbloods are the first opponents with upgraded cards, it could serve as a nice tutorial.
Also beating each FG once should definitely be a quest. But that should have a higher reward than only one upped card.