If a lefty teaches himself (before or after his bar mitzvah) to write with his right hand
What happens with teffilin?
What if he teaches himself to prefer his right hand for everything?
I have two opinions for you:)
My fiend's son broke his right arm and had it in a cast for a very long time. He started getting used to using his left hand for everything, writing, throwing, everything.
By the time his cast came off, his left hand was permanently stronger and he was behaving entirely as a lefty. This all happened when he was twelve. Then comes Bar Mitzvah time. What to do.
I was asked to be the messenger and ask Reb Dovid Feinstein the next time I saw him. His answer was that the boy was still considered a righty. He explained his logic as follows. A person who has his right arm amputated does not become a lefty just because he starts to use that hand now does he? Will you pattur him from putting Teffilin on his left arm now? Of course not! He is still a righty and must continue to wear Teffilin on his left arm. So too retraining oneself does not change anything.
Wait, there's more. This family has close ties to Rabbi Siff, also from the lower east side. They told him the story and he ruled that he believes that the boy is now a lefty, and should wear Teffilin accordingly on his right arm.
The end of the story is that this is in fact the ruling the family went with due to their relationship with Rabbi Siff.
Unfortunately by checking off my answer you've made me feel compelled to provide more than just a story.
In reality this is an old machlokes. See in siman 27 the Mishna Berurah and Biur Halacha that Fred quoted in his comments who says in the name of the Magen Avraham who quotes a Beis Yosef quoting Mordechai that says one who trains himself to be a lefty is treated as one. The Magen Avraham though points out that the Mordechai brings two opinions.
The Aruch HaShulchan in that siman siff 17 brings this issue and says it's a machlokes. In the parenthesis he mentions that he thinks the Ramma's opinion in the Darkei Moshe is that switching usage will change the halacha. He is apparently referencing the Darkei Moshe Ha'Aruch #6. The funny thing is that the footnote there brings a different girsa in the Mordechai that read one is only a lefty if he is born that way.
One more interesting point is that the Aruch HaShulchan says there is a kol shekein that someone who became ill and lost use of his right hand would certainly be a lefty. He also quotes an anonymous opinion of someone who wanted to compare these cases to someone who loses their hand but dismisses this as irrelevant. This is the opinion of the Biur Halacha.