Only #2 brother is left-handed among my family, but he learned to use right hand as well at almost anything from baseball hitting and pitching and writing and drawing when he was very young. It's difficult to notice in most parts of routine daily lives that there are actually so many left-handed people just as it is difficult to notice there are speakers of some regional dialect of the national language. I don't speak dialect except with brothers and parents. Here is a song for the left-handed.
It can take a few seconds to a few years for an indvidual human being to change habits, but it takes more than a few generations for societies to do that.