This is a question that’s been bothering me for a while. As far as I have been taught with regards to this area of history, we go from muskets/later on rifles, lining up side to side, to the more modern fire-and-manoeuvre type of war. But I’m confused about this switch. The two styles are radically different, and I understand why both of them were utilised when they were.
How is this sort of change implemented? There must have been a point where some soldiers would go to sleep in their barracks one night, and wake up to be told that everything they have been taught about infantry fighting is outdated, and they have to use these new tactics now? Are there records of soldiers finding it hard to adjust?
The shift isn't an overnight development.
Light infantry were fighting in open order since the mid-1700s. The French in particular, used large numbers of skirmishers in open order during the Napoleonic Wars.
Initially, this skirmishing is done largely by specialized units (like a French Voltigeur battalion, the British 95th Rifles or the light infantry company of a British Regiment of Foot in the American Revolution or the Napoleonic Wars).
As time goes on, regular infantry start doing more and more skirmishing as well. American and European regiments that otherwise fought in a dense line would often throw out skirmishers ahead of the main formation. This is frequently done during the American Civil War, for example.
By the late 1800s, when fighting in "extended order," "skirmish order," or open order becomes the norm, the change, isn't really that dramatic or abrupt. By the time tactics shift in the 1870s and 1880s, fighting in a more spread-out manner is something many soldiers had already done in combat or in training.
Plus, "stand shoulder to shoulder and shoot" isn't wildly different from "stand 5 paces away from each other and shoot." Soldiers didn't have quite as much to adjust to.