Hello Judy, Your ancestor enlisted in Company E in the 3rd Regiment Tennessee Mounted Infantry at age forty-five as a private at Knoxville, Tennessee on August 11, 1864 as stated on his muster roll and had been enrolled in Blount County, Tennessee on August 4, 1864.
The War Department made a correction error in 1897 stating that he had actually enrolled on July 15, 1864.
His widow filed for a pension on November 8, 1890 application #481.383 and received a Federal pension via the State of Tennessee Certificate #362.498.
Depending upon how much you will like additional information you answer may be at the National Archives in Washington, DC. Many times there is a wealth of information. I've saw some pension files as large as a major cities phone book and still others with only half of a page. I would venture go guess the average is from ten to twenty pages. The chances are in your favor that additional family information will be gained; in my opinion only.
I see nothing to indicate that he had been taken prisoner at Vicksburg or had ever been a galvanized Yankee. Some galvanized Yankee did change names when switching sides but without knowing more I would not want to guess. I believe I can say that some of the future galvanized Yankees at Fort Delaware had been taken as POW's at Champion Hill and Big Black River Bridge which were both battles during the Vicksburg Campaign.