If their views change to the extent of them doing a complete or partial about-face from what they thought of the world.How much does someone have to change, or in what ways does someone have to change for them to become 'a different person'?
It's so obvious that this is about homosexuality, considering your little side note, but, sure, let's go to another extreme and get about answering the question that way.If your son/daughter were to experiement with activities that you found controversial(not affecting their physical health negatively), would you try to stop them and how would you go about doing it?
For example, my son/daughter were to experiment with serial murder. It is not harming to them physically, considering s/he would probably pick off people who are generally weaker or has some kind of psychological fault s/he can then exploit to further strengthen his/her position as the killer.
However, I would still stop him/her, seeing as
1) It is harmful to another person
2) It is morally reprehensible
3) It is a complete waste of time, with no measurable benefit for either him/her, the person s/he is killing, to me, or to society as a whole.
How would I stop it? First off, I'll try to dissuade him/her from doing it again [that's daddy's job].
If s/he doesn't listen, and tries to threaten me in response, then s/he's dead. No questions asked.