Note that first you need to create a variant of the existing model, then you need to switch the model or variant that your factories are building. As I understand it, you can have one factory still building the old model and another switched over to the new variant, then have a third factory producing the next tech level model or variant when it becomes available, each of which will require its own stockpile.
I think the ultimate question is: can we produce and stockpile as many different models and variants as Germany did historically? I can't even venture a guess as to how many that would be, but just the number of different tanks has to be staggering. The Panzer III went all the way through the "M" and "N" variants. Add in all of the Jagdpanzer and Panzerjaeger variants, assault guns, self-propelled artillery, and self-propelled AA types, as well as command and recovery variations, and you're into several pages. Picture trying to stock parts for all of that, and it becomes pretty obvious why Germany had supply issues, even if they could have upgraded the rail systems and provided fuel to transport everything.
