Most of the POW's captured at Island #10 & Ft Donelson were released at Vicksburg in Sept 62, and sent at once to Jackson, Ms to reorganize and refit for active service.
Due to Van Dorn & Price's campaign against Corinth, these exchanged pow regiments were quickly rushed north to reinforce Van Dorn.
The 50th Tn got orders on Oct 7, to prepare to move north and began their movement on Oct. 8. Arriving too late for the disastrous battle at Corinth, these troops met the retreating army of Van Dorn at Holly Springs.
So, the 50th Tn and most of the other exchanged regiments, were indeed in the Holly Springs area even before Oct 27.
Gregg, recently promoted to brigadier general after his release from prison, had replaced Lloyd Tilghman in command of the returned prisoners and their reorganization and exchange process.
I believe the prisoners referred to at Bolivar had been recently captured during the Corintb campaign in early October. Gregg would have been responsible for handling these parolees.