I suspect that you will find a number of Arkansas whites who left the state and ended up in various Union units, but not necessarily "switching sides." I say that because of the lesser secessionist sentiment in some areas, and how devastated these same areas became with various armed forces tramping through them. A number of families in Missouri and Arkansas were displaced by war and might have been in the Indian Territory, Missouri, Kansas, Texas, etc. The direction doesn't necessarily confirm their individual sentiment, nor does prior Confederate service because of conscription.
For example, with respect to sentiment, if one was neutral/Unionist, but was conscripted into the CS forces, it would not be unreasonable to desert and end up in some sort of Federal service in another state.
Challenging project, I wish you the best with it.