Your old cdkey will work with the version from Gog if you can find the installer. I had trouble without login to my gog account but its out there, I just didnt dig. This connects to the bnet/gog servers for multiplayer. Far as it stands from other posts, if you want to launch the game from the bnet launcher you have to pay up, again. I tried to do the same as you Lacroix. I own 2 physical copies and now 2 digital copies, gog and bnet. 
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