I am sure this question has been answered before but here goes.
If awarded an NROTC scholarship and then you ship off to that college and enroll in the NROTC can you somewhere decide you want to be a Marine instead? Vice Versa.
If you have accepted a Navy ROTC scholarship and decide later that you want to switch to Marine Option you can try. You have to apply for a Marine Option Scholarship and they are not easy to get primarily (I think) because there are so few of them. My D's is in school with a girl who has applied for the 2nd year in a row to get one and Marine's is what she wants so badly. Passes the CFT with no problems, etc, etc, etc but still couldn't get it this year. So- if you know that is what you want- I'd start out that way.
As for going the other direction- I really don't know.
I agree with kgrmom - if you really think you want Marines, go for it from the start. It is true that there are fewer Marine option scholarships awarded than Navy ones - I don't have the exact numbers but I was told by the NROTC office in Pensacola that there are thousands available for Navy and hundreds for Marines. In our own multi-state region, 18 were awarded out of 54 who qualified.
The best way-and the most difficult-is a 4 yr. scholarship. Good luck, as there are very few-only a few hundred get accepted each year and those that aren't used do not get regenerated. The Marine Options are part of NROTC and are considered mid shipmen and some of their courses are the same, but they are different-Especially in the PT department. It is easier to switch from Navy to Marine Option if you are in NROTC already. However, it is doable if you are in another ROTC program-provided you have what they are looking for (GPA, PFT etc). As far as going the other way (to Navy) I was told the Marnies do a pretty good job at screening their candidates, so it rarely happens. As you know you cannot apply for Marine Option and Navy---it is one or the other.