George,
My stepfather's great grandfather, Nathan B. Holder,was originally a member of Company G of the Arizona Brigade of Cavalry and later 'attached', according to his service records, to Co. C. of the 7th Texas Cavalry. He was first captured at New Iberia, La.,Dec. 21, 1863. After being exchanged he was captured again at Camp Pratt, La., on Nov. of the same year. Apparently he was exchanged again because he was captured for the final time on Aug, 5th, 1864 at Cross Bayou, La. He died as a prisoner of war, March 1st, 1864 at Elmira, NY. From a newspaper article, it was stated that he was also wounded 3 times..(this might be sensationalized). His records indicate he was wounded at Yellow Bayou, May 18,1864. I have no idea if the transfering of prisoners taken West of the Mississippi was a common practice, but thought the story interesting and might be helpful