If I am in the army reserves and join ROTC in my college, will I have to change my MOS ? Or will I have a option to change my MOS?

I should add that just because your MOS changes doesn't mean your actual duties at your unit changes. That depends on your actual leadership. I saw cadets who spent 2 years at their units continuing to act like privates. Other took leadership roles. For me, I immediately took the XO position then went with S6. It was all for the purpose of training and mentorship. I had no real responsibility in the legal sense of the word. The only duties I had were specifically outlined by the CMDR and my authority was almost non-existent.

Actually, there are several benefits to joining ROTC prior to the junior year. First, a cadet gets to know his/her cohort and they can all start working together as a team early on. Second, BCT/AIT can give you credit for the first two years of ROTC but they are not true equivalents as the focus of BCT/AIT and the ROTC basic course are vastly different. Third, every cadet should know what they are getting into before they sign an ROTC contract. If a Soldier has not participated in ROTC until their junior year, they may not be prepared to step into the role of an MSIII cadet, especially if their only experience is BCT/AIT and 1-2 years as a reserve component Soldier. Finally, getting involved early on gives the PMS and other cadre time to assess the Soldier's potential to become an officer. As a ROO, I encourage all potential cadets to get involved in the program as early as possible.