So what is the deal with Japan infantry squads? Some units use device 707, which do not upgrade, and some device 709, which actually downgrade at 4301.
Also, why SNLF, and para squads are so weak? Why there are no upgrades at all for Japan infantry?
I'm guessing part of the reason is the cost.
INF upgrades are really expensive as opposed to other devices as (per the manual) the old INF units are returned as MP. So you have both the HI cost of building the new ones (6x load cost in HI per squad) plus the supply cost (load cost). That's about 30,000 HI and 4500 supply per division, IJ has about 200 divisions, so 6,000,000 HI and 900,000 supply ... you need a new economic balance to allow for this ...
Someone tested it in WITP, and in fact, upgrades took less, than manual claimed. Probably situation in AE is similar.
There are, like 100-200 Para squads in Japan army, and 1000-2000 SNLF squads. That would be quite cheap, yet they do not upgrade either.