I sit possible to join the NROTC at a 4-year university after attending a 2-year college?

I know most NROTC students join the program their freshmen year and serve the full four years. My question is whether or not a student can join upon transferring from a 2-year school to the 4-year university or does he have to join and serve the full four years. At the moment, I am not in physical shape to join, but am working on being so in the fall of next year. Also, I have made the mistake of many college students and goofed off during my first years at the school acquiring a few withdrawls, retakes, and only one failure, with the rest of my grades consisting of mainly A's with one or two B's and I plan on making up the failure. Most likely, I will still attend the 4-year school either 3 or the full 4 years before graduation, due to my goofing off at the 2-year school, but I have straightened up and am focusing on my schooling acquiring good grades and holding a GPA of 3.1 and rising. I hope to join as an officer upon graduation and, hopefully, serving aboard a submarine. Any help would be greatly appreciated, thanks!