From: Toronto and Lima
In the current game, once you get a good starting model design (greater than 100), you can pretty much can keep it forever, just updating it
That means that your 25mm gun, 5mm armor, light engine light tank can suddenly become a 60mm gun, 100mm armor, heavy engine; this is kind of unrealistic because if you are changing everything then you are actually changing design
what I suggest is to lock some of the decisions for the next version, maybe you cannot change the engine, or maybe you are forced to the previous and next alternative (for a high velocity gun, with 40mm as example, it means you can go to 25mm or 60mm in either howitzer or high velocity).
Otherwise what is the point of changing to a new model?