Do I eventually have to move units?

You may need to move units at some points in your career such as if you are promoted and there are no available positions in your unit at the next higher rank. You may also want to move units if you change your geographic location and want to attend Battle Assembly closer to your home of record. Most Army Officers are encouraged to change units every 2-3 years. 38Gs are not required to but can change units if they so desire. There are limited 38G positions available within the US Army Reserves so there are limited units to experience.