How did the United States military change between the Mexican American War and the Civil War?

The Civil War was a turning point in United States military history, both technologically and administratively. From that perspective, it represented a significant advancement over the war which preceded it.
Technologically, the Civil War was fought with more advanced firearms, which afforded vastly improved accuracy and range, and this resulted in much higher casualty rates. In addition, the difference in manpower requirements should be noted: according to PBS, in total, only about 100,000 soldiers served in the Mexican War for the United States. The Civil War required a much higher commitment of manpower: according to NPS, over 2.5 million soldiers served for the Union, as opposed to somewhere between 750,000 and over 1.25 million troops for the Confederacy.
In addition, we can note bureaucratic innovations. The Civil War was the first war for which the United States instituted conscription. By contrast, the Mexican War was fought with volunteers. We should also note that it was in the context of the Civil War that the income tax was introduced to pay for it. These examples suggest a significant degree of political centralization relating to U.S. politics in the Civil War.
https://www.nps.gov/civilwar/facts.htm

https://www.pbs.org/retired-site/