Historically, when the US Army regulations changed from the dark blue to the sky blue cloth for trousers in December, 1861, the troops were wearing the dark blue ones. And those on hand, or in store, were still worn and issued until exhausted. After December, 1861 the Army only produced or contracted the sky blue trousers, and these were issued once available in quantity after early 1862. The dark blue trousers were yet to be seen, here and there, to the close of the war.
From mid-1862 in the Peninsula, the Prince de Joinville illustrates US Army regulars yet in the dark blue trousers.
View attachment 533642
From May, 1862, men of the 5th New Hampshire on the Peninsula. Mix of dark blue and the new sky blue trousers.
View attachment 533641
From late summer 1862, the 41st New York at Manassas.
View attachment 533632
August, 1862, Union infantrymen at Manassas, the chap at right in dark blue trousers.
View attachment 533633
But even after 1862, it appears the 1858 pattern trousers were yet seen in the ranks, here and there.
April, 1863, two of these US Army engineers still wearing the dark blue trousers.
View attachment 533634
From the late summer of 1863, US soldiers at Charleston, SC. A couple of these fellows in the dark blue trousers.
View attachment 533635
Also at Charleston, August, 1863, the men of this battery of the 1st US Artillery yet wear the 1858-61 pattern dark blue trousers.
View attachment 533636
From August, 1864 at Petersburg, VA, two of these Army engineers yet sporting the dark blue trousers.
View attachment 533637
Among Sherman's men, one of his bummers at Atlanta in September, 1864 yet in the dark blue trousers...
View attachment 533638
From 1865. Some of Sherman's soldiers at Fort McCallister near Savannah in early 1865. Some in the dark blue trousers.
View attachment 533643
Some Union soldiers in captured Fort Fisher, North Carolina, in early 1865, with a few yet in the old pattern dark blue trousers.
View attachment 533644
Among some Army of the Potomac men photographed at Appomattox in April, 1865, a couple yet sport the dark blue trousers.
View attachment 533639
View attachment 533640
History aside, as mentioned, different reenactment clubs and event rules have different standards, and may require sky blue trousers for Union impressions.