I am still behind the Padres (although they pretty much stink so far this year!) even though I have lived in South Carolina for several years. James is from an area without any professional teams but is a 3rd generation Chicago Cubs fan. We both root on the Padres and the Cubs, but when they play each other it's "every man for himself!" LOL Truth be told, however, I keep up with the Cubs more than the Padres now (I know most of them by name but very few Padres).
We don't have a professional baseball team near us now. I'm not into football, but if the Chargers or Panthers are in the Super Bowl, I watch. The Chargers because I am from San Diego, and the Panthers because they are local (and they have the prettiest uniforms! LOL).
As far as the original question, I think most Americans stick with the team they grew up with if they move. I know some people who would probably trade their family members in before switching team loyalty!