If you change your regional setting before you start the game, you will be able to get that region's Vivillon pattern. However, once your game has started, changing location doesn't work for getting another Vivillon pattern. You have to restart the game to get another pattern.
Currently, I don't know of any negative impact changing your 3DS location has upon your game/ DS, but I imagine it might affect your connectivity to Dream World or Pokebank temporarily later on. Your 3DS will most likely need a day to adjust to any system changes before being able to connect to the internet, as it did in Gen V when you changed time/ date.
Also, if you're worried about this being cheating, it isn't. Considering that you have to restart the entire game just to get a Vivillon pattern and the fact that your 3DS even has a regional setting for you to change, meaning you didn't hack into your 3DS and forcibly change your region, means that it's been designed that way. It might be a loophole, but not cheating.
Two (Game FAQs)