I believe that Fort Leavenworth was used as a P.O.W. camp during the War. However, it was a long way away from the Virginia theater of war to be shipping POW's.

It was fairly uncommon, given the numbers of prisoners taken, for a southerner to "galvanize" and join the Union army to excape prison, But it did happen. Usually those southerners who did so were shipped west to fight against the western Indians. So Fort Leavenworth would have been one of the places that such Galvanized ex-southern soldiers would have been sent.

Another thing to remember is that during the war there was no right to choose ones side.

If you were a southerner in a Union Uniform you were subject to being shot as a desserter, or a traitor, if you were even captured. That is one of the reasons that such galvanized southerners were shipped west to Fort Leavenworth. So a galvanized southerner would have been very carful to protect his true place of birth if caught it such a situation. If you haven't checked his CMSR I would think this would be an important step to taken. However, be aware that IF he did galvanize, that there will probably NOT be any connection made in his Confederate records.

In 1862 both sides were still exchanging prisoners. So galvanizing wasn't done as much as it was later in the war, when Grant ended the exchange of prisoners. How this ex-confederate captured in Virginia, even if he did galvanize, which he would have had to have done, became connected with the 1st Indiana Cavalry would probably make a pretty interesting dime novel story. I would say that the odds would be unique almost one of a kind.