I don't know if it's possible to change sexual orientation, but I'm not willing to rule it out. I think the ironclad "sexual orientation cannot be changed" position is more a political opinion than a scientific one. It's not in the gay community's best interest for there to be a way to change orientation. After all, it's not a large leap from "can change" to "must change".
There are several psychological phenomena which make me think that it's at least possible. Multiple personality disorder, hypnotism, and amnesiacs that completely change personality all suggest that there might be a way.
I don't really think it's worth any effort to find out, and I certainly don't think anyone should be made to. It might even be dangerous. But it's enough to make me think any blanket declarations that it's impossible are blustery attempts to stake out an identity.