Nice photos.
Believe you are correct. The 38th MS Cavalry was initially formed as the 38th MS Infantry during the summer of 1862. After the infantry unit was captured in Vicksburg and exchanged, at the end of 1863 it became mounted and reassigned as the 38th MS Cavalry.
See NPS battle unit details extract below:-
38th Regiment, Mississippi Cavalry
OVERVIEW:38th Infantry Regiment was formed during the summer of 1862 with men recruited in the counties of Holmes, Tishomingo, Alcorn, Wilkinson, Hancock, Harrison, Pearl River, and Marion. The unit fought at
Iuka with 322 men, then reported 35 casualties in the Battle of
Corinth. Later it was assigned to General Hebert's Brigade in the Department of Mississippi and East Louisiana. In July, 1863, it was captured in
Vicksburg and during the siege lost 35 killed, 37 wounded, and 2 missing. Exchanged, the regiment contained 24 officers and 115 men in December, 1863. It then was mounted and assigned to Mabry's and W. Adams' Brigade, Department of Alabama, Mississippi, and East Louisiana. Continuing the fight in Mississippi, it sustained 74 casualties at Harrisburg. The 38th was included in the surrender in May, 1865. Its field officers were Colonels Fleming W. Adams and Preston Brent, Lieutenant Colonel Walter L. Keirn, and Majors Franklin W. Foxworth and R.C. McCay.