Heavy tanks are brigade attachments. They will only upgrade to improved units when you research the appropriate heavy tank technologies. The brigade will be upgraded.
Light tank divisions are automatically upgraded to level III medium tank divisions once you research the level III medium tank technology. These upgrades are done in the same way that all other upgrades are done, through the upgrade slider in the production screen.
Once you have researched the advanced light tanks technology, you get the option of a new brigade, the light tank brigade. In this respect, the light tank brigade is treated exactly the same as the heavy tank brigade, except it has no natural upgradibility.
So, to recap, it is a game design feature that light tank divisions are upgraded to medium tanks once your technology level reached level III medium tanks.
