You can do College Program all the way up to the end of your Sophomore year. After that, you either have to be on a scholarship or be an Advance Standing student in order to get a commission from the NROTC (not counting STA-21 or MECEP students). If you join the NROTC as a Sophomore, and get a scholarship or Advanced Standing, you can make up the NROTC classes in order to graduate on time.
My info may be dated as I finished my NROTC job back in 2004, but we had 3.5 year PNS, 3 year and 2 year scholarships as well as Advanced Standing opportunities.
If you do the transfer option, you have to hit the ground running; grades, and active participation in the NROTC will get you there. It has been done before, but you have to earn it.