A few observations about terrain in China after sorting through the maps and the rules of the old GR/D board game War of Resistance, about the fighting in China from 1937 through December, 1941.
In AE, most of the terrain in the large box Shanghai-Nanyang-Peiping-Tiensten is 'cultivated' and open for rapid movement by the Japanese, especially with armor.
WoR has terrain elements that significantly slow movement, and make the area much more defensible.
Both AE and WoR account for the new course of the Yellow River, after Chiang Kai-Shek blew the dikes to flood the countryside and block the Japanese (at the cost of killing tens of thousands of Chinese, and making hundreds of thousands more homeless). In AE, the hexes on either side of the river are cultivated, and there are no obstacles other than crossing the river itself.
In WoR, the hexes are swamp on either side of the river. The AE equivalent would be swamp hexes starting in Chengchow and Kaifeng, and running side-by-side along the river SE (in the game) through Pengpu then east through Hwaiyin to the sea.
Between the Yellow and the Yangtze rivers, nearly half of the territory covered by 'cultivated' hexes in AE, are 'irrigation intensive' in WoR. These are treated as swamps during the rice-growing monsoon season from May-September. During the rest of the year they do not limit movement, but armored combat is restricted. WoR has other 'irrigation intensive' regions throughout China, notably including most of the Chungking-Chengtu basin.
Adding the "Yellow River swamps" would more realistically (IMHO) restrict offensive operations; the Chinese could anchor a defense in hard-to-flank Chengchow, in x3 terrain across a river.
I don't know of a practical way of representing all of the seasonally flooded regions in AE, if a modder chose to do so. But adding three more swamp hexes -- 86,46; 87,46; 86,47 -- in the 'intensive irrigation' triangle between Sinying, Nanyang and Chengchow would capture the spirit of how the rice paddies canalized the direction of advance in China, and limited both Japan and China's offensive options.
I too appreciate this insight into these aspects of the Chinese area of the game. Let me say that I think the most important word in your question is "practical". There are methods available to achieve what you seek. The practicality of each is very subjective in that considerable effort may be required. So, as always, we have a cost-benefit tradeoff that must be addressed.
There are at least two ways to get the map to change as you play the game.
1. You can manually edit the map data file (pwhexe.dat) and swap the file in/out at the appropriate dates. If you are playing PBEM, both players need to do so in sync. To edit the pwhexe.dat file you need an editor. There is one available at:
which I wrote. It is very much a bare-bones editor and there is a considerable learning curve but I think it is serviceable for such a purpose. Please note that this will change the underlying data but not the map art work. You could also edit the applicable art panels for swapping concurrent with the data file swap to improve the aesthetics, although the game play would remain the same with or without art changes. I of course will answer questions regarding the pwhexe.dat editor. Best way is via PM.
2. Quite a while back I created a utility (Launcher) which uses a script-based method for changing the map based upon the game date. It is possible to think of adding the changes you describe to the script. I must confess in all honesty that I am not motivated to take this on at this time. In my estimation very few folks have used the Launcher, although the feedback from those who have has been generally positive. I have had a fuzzy notion floating around in my head for some time of updating and improving the Launcher but absent any real sign that it might actually get used I doubt I will go forward with this. It would be possible for you to edit the script and I will answer questions if you want to take that on, but be warned that editing the script is likely to be a steeper learning curve than with the editor described above. The Launcher is available at the same link as the editor.