A lot of people simply don't have a dominant hand. They're typically grouped with the people for the hand they write with, but that's a pretty incomplete picture. I suspect your friend who "switched" never had a dominant hand in the first place.
I'm one of those folks. I write left handed, but other stuff tends to be mostly, but not completely, right handed. This is mainly since there's a small advantage to right handedness (baseball gloves fit, mouses are there, guitars are right handed by default, etc.). There's a lot of randomness: I eat right handed, except when using chopsticks (left-handed), including long cooking chopsticks, but tongs (which have a similar purpose) are with the right hand.
Assumed unrelated, but responding to a different part of your post: I can also read and write in mirror image with little loss of speed. I've never thought of that being particularly special. I just started randomly taking notes in mirror image in high school out of boredom (and right-to-left is slightly more comfortable for lefties).
Switching hand to right hand only was definitely forced in the Irish education system back in the 50s/60s, I have older relatives who were forced to switch hand.
There are also people who completely ambidextrous, look at the professional snooker player Ronnie O'Sullivan on YouTube. He plays against people who would have put in equal amount of time tin the 10s of thousands of hours and no other professional player can play as fluently on either hand.
I'm one of those folks. I write left handed, but other stuff tends to be mostly, but not completely, right handed. This is mainly since there's a small advantage to right handedness (baseball gloves fit, mouses are there, guitars are right handed by default, etc.). There's a lot of randomness: I eat right handed, except when using chopsticks (left-handed), including long cooking chopsticks, but tongs (which have a similar purpose) are with the right hand.
Assumed unrelated, but responding to a different part of your post: I can also read and write in mirror image with little loss of speed. I've never thought of that being particularly special. I just started randomly taking notes in mirror image in high school out of boredom (and right-to-left is slightly more comfortable for lefties).