It is often assumed that U.S. infantry in frock coat and regulation hat were likely to be "early war" and those in the sensible sack coat and forage cap to be mid-to-late war. The reality was actually more complex.
Take for example the 104th Illinois Volunteers. The Company of Military Historians plate series (Military Uniforms in America, Plate No. 388) depicts the uniforms of the 104th Illinois, which served in the Army of the Cumberland and saw action at Chickamauga, Missionary Ridge, Buzzard Roost, Resaca, New Hope Church, Kenesaw Mountain, Peach Tree Creek, Utoy Creek and Bentonville, suffering 405 casualties during the war.
Their first uniform, received at Louisville, KY in Sept. 1862, was the Illinois state-issued uniform — dark blue forage cap, sky blue trousers and dark blue jacket with shoulder straps. The Jacket was trimmed with sky blue piping around the collar, cuffs and shoulder straps.
In Dec of 1862 the regiment was surprised and captured by John Hunt Morgan's command at Hartsville, TN, suffering 30 percent casualties. After being paroled, the unit was reorganized and returned to duty in April 1863, this time wearing sack coats and forage caps. The caps were soon replaced by hats, however. Their new brigade commander, John Beatty ordered his regimental officers to requisition regulation hats for all enlisted men to maintain uniformity throughout the brigade: "All white hats, citizens clothing, and un-uniform clothing will be collected and burned." A tailor in the 104th "lowered the regulation felt hat, to a one story affair" for some of the men.
By the Atlanta Campaign of 1864 the sack coats had been reduced to tatters and had been largely replaced by regulation frock coats. Photographs indicate that many of them had the standing collar lowered, presumably by tailors in the ranks.
Jackets to sack coats to frocks; not the usual transition, but perhaps not at all untypical.
Another note about jackets: Photographs of Missouri volunteer regiments raised in 1864 show them in plain blue jackets without piping. These were worn in the final years of the war rather than the early years. Go figure