If one is "born left handed" and for convenience learns to be right handed instead, except for writing. do the muscles and body in general have a predisposition for being left handed that switching to right handed (early on) would conflict with?

Reinforcing a specific handedness, can stimulate neural pathways which then aid in "muscle memory" when using that hand. If you are naturally left-handed and use your right hand more, you will reinforce the brain pathways that support right handedness. I would recommend alternating use of both hands so that you maintain good neural connections in both sets of pathways.