I disagree from the American side.
Everytime I look up division composition of American formations from Overlod to the end of the war (Europe), I see every couple of months battalions being swapped out for different types. Tank destroyer battalions in particular got swapped around all the dam time. It makes sense given US doctrine.
I know you ate focusing on artillery in this discussion, but I see no reason any battalions can’t be swapped out with some frequency.
Yes, if you look at the amount of paperwork produced by the various general staffs - it is staggering. Maybe it is good that they at least did not have powerpoint and excel back then.
However, there are perhaps two details worth keeping in mind. First, at the level of abstraction of HOI4, the US infantry division kept it basic structure of three infantry regiments of three infantry battalions, and an artillery regiment with one 155 mm and three 105 mm battalions throughout the war. And many of the changes that occured post-war (such as replacement of the 105 mm regimental cannons with 4.2-in mortars) do not currently have an HOI4 representation. But the TD battalion was never an organic part of the infantry division, but rather an attached unit - although as you point out, the attachments in practice became essentially permanent. But while the US TD battalions were entirely SP, whereas other armies relied mostly on towed AT guns (the Germans and Soviets added a company of SP AT towards the end of the war, and the British experimented with all kinds of configurations), it would still seem reasonable to see all these differences could be seen as variations of a generic "AT battalion" that most divisions had?
There were some examples where infantry divisions were significantly re-organized, but my impression conveyed above was that these were relatively rare. The US pre-war reorganization was one, but perhaps the most striking one was the German one from late '43 / early '44 where the number of battalions per regiment was cut from three to two, but an independent battalion was added to work more directly with the newly formed StuG (SP AT) company in the AT battalion, while retaining the artillery regiment with one 150 mm and three 105 mm artillery battalions. Still, even this would in HOI terms only be a reduction of the number of infantry battalions from 9 to 7...
A larger variation could perhaps be seen in the armored formations, where allied armies initially did not get the tank infantry balance quite right (too many tanks), while the Soviets struggled with coordination of larger formation and deployed most of their tanks in small 65 tank brigades, a few of which were eventually grouped into "tank armies." The Germans, on the other hand were hard pressed both in terms of tank production and availability of fuel, which meant that they had to find the most "cost-effective" solution rather than simply the most effective one. The result was quite interesting - with one tank regiment of two large battalions with 91 tanks each (one with Panzer IV and one with Panzer III, later replaced by Panthers), and two infantry regiments with two battalions each. One of the latter was mechanized and followed the tanks, while the other one was motorized and held ground. But in HOI4, both the Panzer IV and Panther are medium tanks, and one cannot currently assign different roles to the infantry battalions, so it would simply be a division with two medium tank, two mechanized, and two motorized battalions. Of course, two tank battalions is not much for a HOI4 armored division, but calling each of the eight companies with 22 tanks each a battalion would be a bit of a stretch - in particular if you would like to compare it with a US tank division with three tank regiments and one mechanized regiment.
Thus, my overall impression is that with the current division designer one cannot easily reproduce the details of the historic formations, and instead should try to make divisions that are more true to the "spirit" of the historical ones. But this makes the idea of detailed changes in composition a little hard to interpret. Well, at least that this would be my 2c.
