No, you must go through your Career Counselor for this because it was a reenlistment contract. All you have to do is ask for the Assignment Manager's "OK" for the assignment ... you'll need something in writing (normally an email).
Here's the reference about reassignment and rank change:
AR 601-280, Appendix E - Reenlistment Option, Table E-5:
Line: 5Item: Option Information
Comment:
a. The period of stabilization will commence upon arrival at the new duty station. If the Soldier is subsequently placed on TDY in excess of 30 consecutive days, the period of stabilization will be extended by that number of days in excess of the consecutive 30–day period unless stabilization is waived by the Soldier concerned.
b. If ARMY G1 determines that, to meet the operational needs of the Army, the unit must be deployed, relocated, transferred, or reassigned from the installation of choice, the reenlistee will deploy with the unit. There will be no grounds for a broken reenlistment commitment.
c. If the Soldier’s rank or PMOS change before reassignment, the Soldier may be ineligible for the assignment for which reenlisting and may be assigned according to the needs of the Army.d. If the guaranteed station of choice is inactivated and transfer of the Soldier is necessary before completion of the 12-month stabilization, the Soldier will be given his or her choice of assignment to any other station in the major command to which assigned if a vacancy in rank and PMOS exists. If this is not possible, reassignment will be according to the needs of the Army.
e. If the guaranteed station is re-designated, relocated, transferred, or deployed, the Soldier will remain assigned to the station and will be moved as part of the station.
f. No promises can be made that assignment will be to a specific unit within the station. Any promises, either oral or in writing, indicating a specific unit, will be considered void.
g. The station includes the installation, and any other location in close proximity, for which the installation has requisitioning authority (that is, Fort Eustis and Fort Story, Fort Stewart and Hunter Army Airfield). Even though these cases are not considered unfulfilled, every effort should be taken to assign the Soldier to the actual contracted location.
h. If the Soldier’s rank or PMOS change before reassignment, the Soldier may be reassigned according to current qualifications and the needs of the Army.