It was not at all unusual for a soldier to serve in more than one unit. Some men were discharged from one unit and subsequently reenlisted in another. Some men were cut off from their original unit and joined another. Some men deserted one unit and joined another. Some men who were on furlough from their unit were detained by higher authority and assigned to another unit. The entry in your ancestor's record about being absent without leave from his infantry regiment and serving without authority in a cavalry regiment, was not uncommon in Arkansas in 1864-1865.