They did this to me. Didn't work, though. Just showed 2 things: I'm ambidextrous and my hand writing is illegible no matter what hand I write with.
Left handed people are rare. Big people are rare. Left handed big people are quite rare. They're also irratating, so they tended to get converted to orthodox back in the day.
I’m ambidextrous. Write left-handed. Throw right-handed. Bat from both sides (below Mendoza lifetime.)
Southpaws have a natural weakness to big right hands. The heavyweight division is often filled to the brim with fighters who have a nasty right cross. We saw what happened to Ortiz and Moore. Maybe because of this, trainers either consciously or subconsciously made the decision to avoid training a fighter as a southpaw even if he was a natural leftie? Plus back in the day it was difficult to market and promote southpaws and other fighters hated having to face them so it wasn't a good financial incentive.
It seems quite common for lefties to play racquet sports right handed. When I play cricket I bowl left handed and bat right handed. Michael Clarke used to and Simon Jones Bowled with his right but batted left. I’m pretty sure Rafael Nadal is naturally right handed, but plays tennis with his left. I just wonder if some of it comes down to most lefties being stronger with their right foot as they are able to get greater power driving through their stronger foot, faster hip rotation etc