How hard would it be to switch to the Infantry?

I REALLY want to become a United States Marine. I plan to join at 19 with a 4 year enlistment. Just in case I do not stay in the Corps I would like a MOS that will allow me to get a job easily and a skill. The problem is that I really want to be Infantry but I know I wont gain any skills to use in the Civilian life. So if I start as another MOS and (after my 4 year enlistment is up) reenlist how hard would it be to switch to Infantry (I would most likely be an E4....possibly an E5 so would I get a demotion in order to get into Infantry?)?