I'm currently in the Army Reserves and plan on contracting in ROTC. After commissioning, can I go active duty and can I change my MOS?

CPT Snell just about covered it. After you commission, you can change your AOC (Officer MOS); but only at certain points in your career. Certain branches such as Special Forces, Civil Affairs, and PSYOPS are not entry level, so if you want a job in those fields you have to request a branch transfer once you meet their eligibility criteria. The same applies if you want to transfer to a Functional Area. Then there are specialties such as PA and JAG where you have the opportunity earn an advanced degree as part of the branch requirement.

You may end up being branch detailed after ROTC: that means that you will serve in one branch for a few years before being transferred to your permanent branch. Some cadets request this to get experience in different fields, and some cadets choose branch detail because they are not high enough on the OML (Order of Merit List) to get their preferred branch from the jump.