But mechanically, how would that giving in be represented? Aspect changes? Temporary aspects?
I think the best is to define a "new" concept: goals.
The DFRPG is written as a mirror to Jim's writing style, so we can go from his
livejournal posts. In a conflict, you have opposing sides and each side has a goal. The side that wins the conflict can achieve their goal, the losing side does not. The middle ground would be a concession, where both sides win (or most often probably both sides lose). If you start out like that, it is much easier to figure out what is going to happen, once the fight is over.
I've just had something like that in my pbp, where a character wanted to interrogate a captured vampire. We started with a social conflict, where each of the opponents tried to convince the other to join his cause (or at least help him). The character won, and then we entered a
Sequel, where the character just asked his questions and the vampire answered, no further roll of a die needed.
In your demonic co-pilot example, the demon's goal would probably be full control of the body to go on a killing spree (though depending on the demon, it might be a little more refined). The host would most likely not want to give up his control, he only wants to use the transformation to kill some drugdealer and be done with it. Now if the host already has some consequences, the demon might just hit him again, when he is trying to do something important, and the only way the host can prevent that is taking an extreme consequence. Of course, you can always push on consequences the has taken to keep control before, which will probably get him to take an extreme consequence much quicker. A concession could be, that the host allows the demon control for a short time, if he also takes care of his goal. The GM would of course have to find a way to make a concession dangerous enough for the player to consider his options, otherwise he'd just always "let the dogs out" and be done with it.
Especially in this case, you can write down the goal of the winner as an aspect. That way, you can let the player keep playing his character, but you can also compel (without a fate point, it's a tag on the taken out aspect) him to play out the demon's goal.