You don`t have to upgrade anything other than tanks and fighters.
Depends on difficulty/other factors. Often, tanks and fighters are the only things that are updated before 1943, If I need to push synthetic techs, and so on.
When Germany faced new Soviet tanks like T-34 and KV, Germans simply put on a bigger gun (and more armor) on their Pz4 and thus could continue to mass produce a very cheap tank while largely neturalizing the Soviet investment into more expensive and heavier tanks that were notoriously unreliable. And then in 1943 had to rush a new line of tanks, because issues cost and reliability issues tend to get sorted out over time.
In practice, however, there were 1 reasons Soviets didn`t introduce newer designs, that they already had: They did in introduce new designs of higher priority, like IS-2 deployment was prioritized over T-43, and IS-2 transition was very hard, reliability and build quality were poor, and it took a tonne of time to sort out.
Then, you might find SVT-40 that was withdrawn for high cost, PPSH and similar SMGs, that were put in service, and of course their air force, that introduced and upgraded 3 main lines of fighter Yak-1 to 3 to 9, LAGG3 to LA5 to La7, and MIG, a vast array of new light armor and armored vehicles, T-60, T-70, SU-76, and various TDs and AGs.
T-34 is actually the exception for Soviets, not the rule, pretty much all other armor, which always get`s ignored for some god known reason was upgraded in 42, 43, and 44, it`s just T-34 hull that was kept through the war, while it`s turret was constantly redesigned.
Yes, same as BF-109, only a few re-engines, and complete change of armament.
Just replace Tigers and Panthers with Shermans and T-34, and T-34 and Shermans for Pz3 and Pz2.
It is just a reality of world, that USA and SU had more industry and manpower, and would produce more out of it.
The reason USA kept building Sherman, I know, shockingly, because it was a 1941 designed tank, and US had to fight with something in 1944, not spend another 1-2 years ironing teething problems of Pershing till war was over. Then, despite popular belif, Sherman performed very well, and there was no reason to replace it till 1944, at which point, as you might guess, such replacement would not be deployed till war was over.
It was actually made more complex, as war progressed.
It might have something to do with UVZ employing 29-31k workers over 1942-1945, while Nibelungenwerke started at 5k in 1941, and topped at 8k, even then it had to do with lost of labor being foreigners and "nonvolunteer" vorkers, as to stay within forum rules, which, obviously decreased productivity.