It's definitely possible and it was relatively easy for me. I was in AFROTC and switched over my last semester in college. I went to the OSO office and got a guaranteed pilot slot, went to OCS last summer, and am now at TBS. I chose to go the PLC route, versus the OCC route because my recruiter said there were about three times as many pilot slots for PLC candidates. The only thing is that I had to extend school for an extra semester. Since it was football season, I had no problem with that. As far as paying back your scholarship goes...you don't have to pay it back if you commission in the Marine Corps. I had a $4500 debt to DFAS and never paid back a cent. I sent them my commissioning paperwork once I got my bars and they took care of everything. It was a pain in the ass, though. You have to work through DFAS, Maxwell, and whatever collection agency DFAS gets to do their dirty work.
Make sure you try to stay on good terms with your PAS. Mine was awesome and I owe a lot of praise to him for where I am today.
Why do you want to switch to NROTC? If go to an OSO, you can sign up and have no commitment in college. Just enjoy your last semester(s) and head to the occassional PT session on Mon and Wed mornings.