Short answers, pulled from the story.
Major General Edward Canby renamed the field forces on the 18th of February 1865. This administrative change transformed what was previously known as the Army of the Gulf into a new entity under his direct authority.
The XIII Corps and XVI Corps were transferred to this Military Division of West Mississippi at that time. These units fought in subsequent battles including Spanish Fort and Fort Blakeley under Canby's command.
Command of the Department of the Gulf passed to General Stephen Hurlbut after Banks left. The military forces in the region saw little action during this period of transition until August 1864 when units began moving again.
Units from the department participated in the land attack at the Battle of Mobile Bay in August 1864. General Gordon Granger directly commanded these operations against Confederate defenses while soldiers moved across the bay area to engage enemy positions on land.
The final major engagement occurred when the army captured Fort Blakeley before reverting to its original name. Afterward, the forces took the title Army of the Gulf once again which marked the end of the Army of West Mississippi designation.