Cryhavoc101
Donor
This isn't practiced even today. Guided shells is still something you have perhaps a few of for key positions but nowhere near enough for a comprehensive breakthrough. Mass bombardment with a certain shell density over a certain period of time would still be the order of the day.
Artillery accuracy was such that (for example) even in WW1 during the later parts of the Somme offensive some German units knowing how accurate British Artillery had become were safer lying up in shell craters in front or behind the trench line rather than in it.
And even before then the French had perfected the art of artillery fire correction onto known strong points to great effect.
The thing wasn't that artillery technology developed that much, it was more that caliber increased in response to greater communications ability and better trucks to move the guns around. Of course as already mentioned by others methodology improved significantly too. Basically the biggest factors in artillery performance was the technologies and methods supporting it rather than the actual weaponry or even shells. You could have super long range artillery (which they did in WW1, after all the Paris gun was on par with the ranges found in the very longest range WW2 super artillery), but without the spotting ability and communications ability to guide it it won't be more than at best long range unobserved harassing fire.
Frankly I'd say that given the technology and methods of the day you'd be better off focusing not on artillery, but mortars for the regimental and below. You'd have the ability to be more accurate and have firepower under the direct command of the infantry who need it and light enough to keep pace with the infantry as they advance. A stokes style mortar in different calibers with WW2 distribution would have a pretty important impact on trench warfare. So what I mean is having a 81mm mortar company at the battalion level, 120mm mortars at the regimental level, and something like the 'short' 81mm mortar or at least a 60mm mortar at the company level.
You'd probably need to develop them more before WW1, which was entirely do-able given that they had been a napoleonic weapon that had fallen out of favor outside of some specialist units like engineers. The high angle of the weapon made it a highly effective anti-trench weapon, while being cheaper, lighter, and generally more effective than much of the artillery of WW1.
Perhaps have the weapon developed during the ACW or have a longer drawn out war of 1870 between France and Germany - Perhaps get the British involved (for example it turns out that Nap III had plans to invade England and capture Portsmouth - the very thing the 'Palmerston Follies' fortifications around Portmouth were built to prevent and they had mortars as part of the defences) - so a drawn out war after a failed attack oin the South coast of the UK could see trench warfare develop and the Mortar become a quick cheap 'howitzer' as you suggest.