I don't claim to know much about this, but didn't Grant stop paroles based on widespread abuse by the CSA? The garrison of Vicksburg was paroled and most of them were back under arms very quickly, as I recall.
Walt
You've made me check the history books. In July 1863 Grant paroled 30,000 Vicksburg prisoners. However, Grant didn't know that "on July 3, 1863 Secretary of War Stanton issued General Orders No. 207, which declared all paroles after this date worthless....Technically the act of declaring the paroles illegal at once set the Confederate prisoners free." (From Immortal Captives by Muriel Phillips Joslyn, copyright 1996.)
The Confederates released at Vicksburg were free to do what they wanted because of Stanton's order. Stanton's order also caused problems for other releases of Southern prisoners that occured after the July 3 date. US General Franklin Gardiner said, "All prisoners captured from Lee's army and impoperly paroled were immediately returned to duty and we could make no complaint." My interpretation is that the Southerners did not have to wait for a specified period before reenlisting because the exchanges were not valid under Stanton's order.