According to his records on Fold 3, Bangs was 28 when he joined the 12th MSM in 1862 and was a native of Oxford, Maine. When the 12th was disbanded and he went into the 5th MSM (2nd Org), it was as a First Lieutenant. His cards show two dates for this--April 3 and May 5, 1863, so take your pick. He was promoted to Captain in Company D of the 5th MSM (2nd Org) effective January 30, 1864 and I believe many men from that regiment including he left the 5th MSM (2nd org) in September 1864 and signed up with the US volunteers 13th Missouri Cavalry; Bangs was captain of company D of the 13th MO Cavalry.
It is important to note that he went into the 5th MSM (2nd org) not the original or "old" 5th MSM (William Penick's regiment), which was ordered broken up in the same order of February 2 or 4(?), 1863, that ordered the 12th MSM broken up. The 5th MSM (2nd org) must have been created at that time; not sure by what order. Based on my research on the 5th MSM (old), even though it was ordered broken up in early February and ordered mustered out entirely in early March 1863 after the men "mutinied" and refused to join other regiments, it actually stayed in the field for nearly 4 more months until late May/early June (thanks to a tug of war between Gov. Gamble and Departmental commander Gen. Samuel Curtis). So the same might be the case in Bangs' situation. It's one thing to issue an order and another to have it implemented quickly, especially when it comes to breaking up commands and shipping various companies in various directions. Hope this is more helpful than confusing.