In April 1863 Joe Hooker was facing the same problem with enlistment of regiments that joined in the first fevers of Fort Sumter, that enlisted for only two years. At the same time the enlistment of regiments that enlisted for 9 months because of the need of the Sharpsburg Campaign of Sept 1862 was also coming due to be discharged.
Several of these regiment mutinied over just exactly when their term of service was to expired. Their impression was that their enlistments expires several weeks earlier than the US war Department said they would expire.
Another thought on these special enlistment could have been an early form of a person providing a substitute. But that may be a little early thinking, because I don't think the Union passed a conscription law until July 1863.