What do I have to do to become a republican?

I foolishly signed up as a democrat when i was young, back in early 2001. Over the years I have become more educated on the issues and realized that I don't share anything with these people. I am from PA and the primary meant nothing in 2004, so I just voted Bush in Nov and never bothered to change it. I voted for Hillary in the primary this year as part of Operation Chaos. How do I finally make the change to GOP? Do I have to go online and re-register to vote? I am in college right now and not near my courthouse, so it would be hard to go there and change my political affiliation.