[chii]

The real question, which is unanswered, is whether it's possible to change a personality when that person doesn't want to be changed.

[Cthulhu_]

I'd say it is, but not by directly confronting them with their personality (e.g. saying "you're a dick" every day), but by more subtle behavioural things (like being nice to them when they're not being a dick).

[bryanrasmussen]

that really depends what you want to achieve, there has been lots of study that shows you can take reasonably well functioning people and break them through repeated stress.