The main tank sequence for the Soviet Union is T24-BT5-BT7-T34 and that isn't a switch from light to medium tanks it is just development progression of tanks.
It would be nice if the divisions structure didn't work like this. The game, by making them a different category, forces them to be treated as something they are not. This is most clear in the German armed forces where pz I and pz II aren't there because they wanted a light tank, they are there as the main body of the panzer divisions. The first generations of light tanks are actually just tanks and the biggest issue is upgrading from light to medium tanks is seen as a division redesign. The biggest issue with light tanks is that they are introduced to the game as a separate category from the very beginning.