I don't know why you'd want to be a leftie, but at least we know the choice is out there if now. The magic causing the change of dexterity is called Transcranial Magnetic Stimulation and is specifically intended for right-handers.
When magnets are applied to a specific part of the brain (the posterior parietal cortex region, which is involved with planning physical movements), researchers at the University of California Berkeley found that their 33 right-handed guinea pig volunteers started favouring their left hands for smaller tasks instead.
While people weren't beginning to write with their left hands, they did find that they started picking stuff up with their other hand, or pressing an elevator button.
The potential here is obviously vast, and I'm not just talking about which hand we use. Imagine if magnets could make us favour healthy food over fattier food? Earlier mornings instead of long lie-ins? I can almost see horror movies being planned using unauthorised TMS technology, that's how big this could get.