From experience - you maintain the same MOS in a transition from active duty Marine Corps to Reserve Marine Corps. Reason: The Marine Corps wants to maintain as high a level of experience as possible in the Reserves in the event they need to be mobilized. If they allowed you to do a lateral move (change MOS) as probably a senior Cpl or junior Sgt, you would be useless to the command assigned in the intell field because you would know nothing about it. They would have to spend dollars to send you to intell school and then you would only be trained to a PFC level of knowledge, not an NCO level. The troops under you would know more about intell than you would - not a good situation to be in.
Additionally, the Marine Corps has been able fill all NCO billets in all MOSs directly from active duty ranks, Hence, no retraining needed. More cost effective and makes the unit more combat ready.
FBI: Unless you get a college degree, as an infantryman, your chances of getting a FBI slot are few to none. They are not looking for Marines or soldiers with combat experience. They are looking for people who have degrees in accounting, computer science, criminal justice, law. If a person has previous law enforcement experience or military background, that is usually a plus but definitely not a guarantee. Two of my officer friends went into the FBI after leaving active duty. One went into counter cyber ops and the other was a field agent. They both already had their degrees.
Intell backgrounds are OK but mean little at the junior enlisted ranks. Officers in intel usually have an advantage but for a specific type of job within the FBI. Some become CIA analysts, but even that area is tough to get.
You will be in the IRR at a minimum after active duty. Drilling Reserves is not a good idea if you go to college in a technical major like engineering, chemistry, math, computer science; others less impact. Reason: If you have exams or lab papers due on Monday or Tuesday, you won't be able to prepare during your drill weekend. Impact is possibly flunking the course!!
My suggestion is try and get intell now and after 4 years, get out and go to college in one of the majors I listed. That will increase you chance of being accepted to the FBI academy where you still have to excel to get your badge. If you are thinking that you can go back on active duty via the drilling Reserves, you need to think again. It is VERY difficult to return to active duty unless you are in a critical MOS. Neither 0300 or 0200 MOSs are considered critical. You can apply for OCS while in college via the Platoon Leaders Class. If you get out of the Corps after 4 years as a Sgt or senior Cpl, your chances of successfully completing OCS is very high. Google Marine Corps Platoon Leaders Class for the principle program that provides Marine officers while they are still in college.
Lieutenant Colonel, U S Marine Corps-Retired (prior enlisted (Sgt); college GI Bill, PLC pgm, commissioned, retired after total 27 years)