One of my problem with the unit research tree was that each and every nation had the generic Type III medium tanks (and strategic bombers also, but for the sake of simplicity let's remain at tanks) and later researched the Type IV medium tank which was a very new model.
However in real history those Type III medium tanks entered service as Type IIIa medium tanks and the last models of them were a much more efficient Type IIIe medium tanks... with a more powerful gun, increased armor, larger fuel tanks...etc.
Indeed the US had chosen to produce the M4 Sherman, the Russians had kept the T34 as their main tank... and upgrading constantly. And those were more or less competitive designs throughout the war. On the other end of the scale there was the UK with the Crusader->Cromwell->Comet->Centurion line.
So I think it would be a nice addition if upgrade will be divided into two different way.
1.) redesignig existing models... this could be easier to research and cheaper to upgrade (since it's field upgradable, the smaller guns should be replaced by the larger ones in the divisonal workshop), but will be much more limited in effect. Also the effectiveness of such upgrades should have a relation with the service history, enemy designs, the technical level difference... etc and the number of earlier redesigns (at a point one would reach the absolute limits of a design).
2.) switching to a new model, which would be more costly in research and upgrade (this is after all a new tank manufactured from screw to screw), but result in a much more capable unit (which could be redesigned later)... practically it's the HOI2 style upgrade
However in real history those Type III medium tanks entered service as Type IIIa medium tanks and the last models of them were a much more efficient Type IIIe medium tanks... with a more powerful gun, increased armor, larger fuel tanks...etc.
Indeed the US had chosen to produce the M4 Sherman, the Russians had kept the T34 as their main tank... and upgrading constantly. And those were more or less competitive designs throughout the war. On the other end of the scale there was the UK with the Crusader->Cromwell->Comet->Centurion line.
So I think it would be a nice addition if upgrade will be divided into two different way.
1.) redesignig existing models... this could be easier to research and cheaper to upgrade (since it's field upgradable, the smaller guns should be replaced by the larger ones in the divisonal workshop), but will be much more limited in effect. Also the effectiveness of such upgrades should have a relation with the service history, enemy designs, the technical level difference... etc and the number of earlier redesigns (at a point one would reach the absolute limits of a design).
2.) switching to a new model, which would be more costly in research and upgrade (this is after all a new tank manufactured from screw to screw), but result in a much more capable unit (which could be redesigned later)... practically it's the HOI2 style upgrade