Can a Negative Sympathy Trait eventually become Positive by spending enough xp?
No, to have a Positive Sympathy Trait, you must acquire it as such.
Yes, eventually a weakness can become a source of strength.
By spending enough xp one can eventually bring a Negative Sympathy Trait (from Realms of Power: Faerie) to 0. At that point can one continue spending xp and bring it up to +1 and beyond -- effectively turning a Negative Sympathy Trait into a Positive one? I can see reasons for both "yes" and "no". I'm curious how you play it (and perhaps how the authors of RoP:F intendeded it).
Well, see, if a character's relationship with Faerie makes him especially vulnerable to iron, I think it should never actually make him exceptionally good with iron. Though certainly he could be able to overcome the difficulty. I suggest another option: instead of buying the same negative Trait positive, remove it at 0 and take a different positive Trait at 0 that is somehow opposed to the negative Trait, and increase that one. So effectively, the character buys off his negative Iron trait, and then transforms it into a positive Silver Trait?
I disagree, it seems perfectly viable as a "faerie story" for characters to become their opposites. Think of the characters in The Wizard of Oz for example (I know it is not a traditional medieval tale). The stupid scarecrow gets brains, the cowardly lion gets courage, and the heartless tinman gets a heart.
I would say that if you can tell viable stories (esp. faerie stories) about characters becoming their opposite then go for it.
That´s also a nice variant. But i think the original idea stands ok as well, in your example it could be that dealing with the problem with iron eventually made the person surprisingly good with it instead... Sounds pretty fine.
Or overcoming their troubles in a big way, lots of old stories about that.
Richard, in Oz, the Scarecrow was always a genius, and the Lion was always heroic in defence of his friends, and the Tin Woodsman always prevents harm to anyone, even insects, because of his symopathy for them
Also, the scarecrow doesn't get brains, he gets a handful of beans poured in his head. The Woodsman doesn't get a proper heart: it's made of sawdust, and the lion doesn't get courage, he gets drunk. The Wizard's a charlatan who has no power to grant any of these things.
As to "Can things become their opposites?" I believe that some can, some can't, and some are just pretending.
Sure, you can interpret the story like that if you want to.
Well, yes, it is a faerie story, not a medical drama.
If the troupe can see an opportunity to tell a story about a character slowly (or even quickly) becoming his opposite I see no reason to claim that the rules prevent it. In interesting stories characters change.
You just described a cognicent Faerie's tale. The symbolic representations of the characteristics acquired through the hero journey were given to the characters. The realization they had them all the time is the restructuring that happens when one falls in love or finds their destiny. It seems as though every event has led up to this glorious destiny! The wizard is a very, very powerful Faerie pretending to be a charlatan pretending to be the Wizard pretending to be hiding... orchestrated the entire thing, a story to be told for ages, genius.
If Faerie's essential nature is that it doesn't really have one, then all these sympathies are kind of accidental. Positive, negative, opposite, schmoposite. Of course this can change, whether through some logical progression of development or a jack in the box surprise from out of the blue. It's all about the box office receipts.
(Whether it's a good idea to allow this transformation through mere xp from a mechanical perspective is a different question.)