"Needs of the Army" will rarely result in an active duty commitment of less than five years, but it does happen - professional athletes, extreme hardship, and yes, times of steep draw-down. With the cost of putting a cadet through USMA, it is an exception to release a USMA grad early.
In the early 90's some USMA grads were able to resign from active duty up to a year early (that's the most I know of), but to my knowledge it was not widespread . Don't personally know of anyone getting out sooner than that due to draw-down, but in those days the Army was offering cash payments for officers to resign. The ones I knew personally were Captains/Majors past the initial commitment. Not sure how it affected other ranks or those prior to the end of commitment.