Maybe I over simplified my sumation.

Grant had trouble feeding his own men so far from his base of supply at Memphis and depended on a thin steamboat supply line to feed both his own troops and those of the navy, let alone 30,000 prisoners and where was he going to keep them until he was able to transport them north and how was he going to transport them? The simplest solution was the practise of parole. Once they were paroled Grant had no further responcibility for their care.

The Vicksburg Prisoners were paroled home on July 9, TO FORMALLY wait for the "Official" exchange which did occur on paper in September. This process was simply a numbers game on paper only. These parolees were on their honor to not take part in anything military until after the formal exchange.

One thing to considers is that most of these old boys needed the time to recover their health from the ravages upon them during the Siege of Vicksburg and their walking home. Plus the Confederates had to be able to reequipe them with weapons and gear before they were to be of any usage. So 6 months would have been a fairly decent turn around time for his recovery.