I was right footed and right handed as the majority of the population pre ms, then ms hit, then had to become ambidextrus because I had no choice. Even now both my hands are relatively weak, same with the feet because of vertigo and unbalance due to ms lesions. Most msers have to make changes according to where is weakest, especially when other symptoms appear. Like eyes, we have to relearn to either use one eye whilst ON is active then if resultant damage learn to decipher the world in a new way. I use prisms as I now have nastygmus.
I think originally we were one or the other but ms makes it impossible to have dominant bits (unless hardly affected) because other parts of the brain tend to try take over and in time we use what we have left.