Taking >50 Casualties in one turn triggers a Morale Check. Taking >100 triggers a Morale Check at Disadvantage. Taking >150, x2 Disadvantage. All Casualties suffered on that turn are counted together for the purpose of this trigger.
Honestly, this does change things quite a bit. So, couple of differences this promotes greater concentration of fire and at closer ranges artillery volleys becomes far more devastating. This is actually quite impactful in a lot of ways, but to focus on artillery and it's effect on positioning: Terrain that provides cover is quite a bit more powerful at providing defense. Where woods previously it just slowed the attacking damage down, now it provides more protection against more difficult shock checks. Just a quick bit of math to show it: A singular field artillery [experienced, mid range, no other modifiers] has a 35% chance of triggering a simply shock check, with no chance of an advanced one. By comparison, 2 field artillery shooting the same unit has a chance of 69%, with a 20% chance of an advanced one. Basically, focussed fire becomes more than the sum of it's parts.
How does this look for defensive terrain? Let's take the simple wood tile in medium range. The single artillery chance of inflicting an additional check drops to 15%, with battery fire shooting only having a 39% chance of a simple morale check and a 3% shot at an advanced one. I honestly didn't expect this, the shock effect makes artillery vastly more impactful against units in open fields and vastly less effective against units in solid cover. Honestly, I like it. Staying in an open field in the range of artillery SHOULD be pretty dangerous, while cover should do more to make units survive. Bombarding a unit out of solid cover becomes a lot more difficult to achieve.
Ok, how does this impact the artillery ambush? Under the old rules, an artillery unit needed at least 3 turns to reposition and shoot again, something that would be changing. Under our new system, artillery could regain their advantage by moving once behind the lines (1AP), setting up (1 AP?) and firing next turn. Horse artillery might become a special case, being able to reposition every (!) turn due to skipping on the set up stage. But to focus on field artillery, the ambush advantage becomes more competitive (1 advantage /3 shots versus 1 advantage/2 shots), but also has a much more solid chance of dealing a shock effect. Those are quite powerful if the enemy rolls with disadvantage (average of 7 [6 stress] versus 10 [4] on a flat dice and become more powerful as stress builds up [44% of routing at -5 versus 25% at flat roll).
Anyways, a battery with both field artillery units having advantage has a 93% of getting a bonus roll, with a 50% of the bonus morale roll at a disadvantage. In terms of sheer rolls, this would 3 normal artillery roles vs. 4.63 shots, but a 50% chance of getting the disadvantage morale roll, which is quite good. I'm not entirely sure if it's better, but it's certainly close to competitive, especially when trying to overcome cover.
Link to my anydice calculations if you want to check my work. The only special thing is including the highest of 0 and damage at the start, since the program otherwise has the habit of modelling missing attacks as healing a unit instead when adding multiple attacks.
Speculating on how this would change army positioning and tactics: Horse artillery would become the absolute queen of artillery units, able to very consistently flank and ambush. There mobility would also be better in another way, due to the facing changes: They could provide flanking fire, while the normal artillery would take longer to redirect themselves towards any flank. This makes them pretty good for supporting an offensive from the flank. Aside from that, standoff ranges would probably be a bit increased: If you can charge from up to 5 tiles away (move, move, charge = 2+2+2), there is no need to put yourself directly into medium range. This in turn would probably create a greater desire for posturing and a back and forth, units no longer need to hug the artillery support to win a battle, just pull the artillery into range once the lines really form. Artillery positioning is less permanent, with them being able to move 3 tiles forward and skipping only one turn. (Honestly, maybe we could even increase their movement speed to be on par with infantry or give them half a movement point more? They are already punished by a bunch of terrain penalties and the set-up action and can be pushed decently well across open fields.) Disengaging is possible, though yields more ground. Of course, this also might allow for a feigned retreat.
In my opinion, this would lead to vastly more emphasis on movement, since both a retreat and advance are far quicker (6 vs. 3 movment, if we commit). Repositioning across an open field is quick, though runs the risk of being caught by cavalry and running into enemy artillery. Flanking and ambushes are quite a bit stronger due to greater morale checks and a slower response to the push via rotation [Hmm, assuming we force a movement action for repositioning when already in melee, this would make the reaction to flanking pusher weaker 2 vs. 3 melee attacks, and opening the line to another charge in the rear if not protected]. This does make controlling and securing potential flanks more important and forces both sides to be more mindful of positions and moves. Scouting ahead becomes pretty important, so cavalry would generally ride ahead to catch units on the flanks. Fits their overall recon role. I have to see the system in action to move beyond speculation, but I think it set out what the incentives we want: A faster style of play, greater emphasis on strong attacks and calculated plays rather than spamming attacks for attrition. Additionally, winning becomes somewhat disconnected from raw damage: It's entirely possible to overwhelm the enemy with very few shots fired, if they are inexperienced or unlucky.
This is nice and encourages clever play, since winning a shorter battle results in less ammo and lives being expended. I would generally trend towards desiring a quick, decisive battle. Long battles gain us a lot of XP for artillery units, but reduce the general resilience of our units more due to loosing XP with attrition (far more meaningful, as the rank reduces the general cohesion damage you take). The changes look quite good in theory and provide reasons for making more interesting and immersive choices during battle. I suppose all that remains is the actual testing in the tutorial, but I am optimistic. Both sides have more data and experience with the unchanged parts of the system and we had some pretty good ideas.