In regard to you ancestor, John Holland--Holland mustered in Co. H, 3rd MSM (Old) at Chillicothe on April 5, 1862. During the overhaul of the MSM, on 4 Feb. 1863 this company was re-designated Co. L, 7th MSM Cavalry. Note that John Holland's name is also found on the rolls of Co. L/7MSM. He was discharged from the 7th MSM Cavalry on 8 May 1863 due to a disability.
In order to get a sense of what your ancestor may have experienced, you might consult with the Official Records of the War of the Rebellion and follow the activities of Co. H/3MSM and Co. L/7MSM for the time periods in question. In addition, while there are no books on the 3rd MSM (Old), there are two books on the 7th MSM--"Keep the Home Fires Burning: A History of the 7th Regiment, Missouri State Militia Cavalry in the Civil War" by Sherman Lee Pompey; and "Suppressing Rebellion: The Seventh Missouri State Militia Cavalry Regiment in the Civil War: a Story of Events as Witnessed Through the Eyes and Diary of Colonel John Phillips and His Men" by Charles L. Scrivner. I have no idea whether either book will shed any light for your purposes, but you should focus on the organization's activities from 4 Feb. through 8 May, 1863. Key in on Company L and/or the name Captain Henslee (aka Hensley), who was Holland's immediate commanding officer in both the 3rd MSM (Old) and the 7th MSM.
In addition, you could also order the military file(s) for Holland from the National Archives. While there may be one comprehensive file for him that encompasses his duty in both regiments, it has been my experience that there may be two different files. Consequently, I would advise filling out order forms for both possible files. In addition, he will also possibly have two separate pension files at the National Archives, both of which will provide you with different information. Expanding upon this thought, since he was discharged for disability it is likely he received a medical pension. The application for it would detail how he came to suffer that disability (perhaps it was from wounds, perhaps it was from disease....). In any event, there will likely be affidavits providing detailed information from both himself as well as his comrades regarding how it was he came to suffer his disability. In addition, as a veteran of the MSM, Holland would also have qualified for a regular veteran's pension. As such, there would have been a second application process. All of this pension data may be contained in one file at the National Archives, or there may be two files floating around. Sometimes you have to play detective to uncover all the files. Like the military files, you will need to fill out seperate order forms. I wish I could tell you one application would get you everything the Archives has, but it doesn't work that way. And with fee's required to obtain copies of each different file, it can become an expensive process. Good luck.