I checked the Buqa-shop and found the following:

“After paying for the purchase, you will be able to download the distribution of the game "Might & Magic: Heroes VI. Shades of Darkness" in your personal account. The distribution can be downloaded either via a direct link or using a torrent client. This version was uploaded on May 7 and contains all the expansions for the game, including "Shades of Darkness". After installing the game from this distribution, you will not need to download additional updates.”

Does this mean that by downloading this expansion, I will be able to play the previous versions as well? If anyone knows, please let me know.