You ARE supposed to be able to upgrade. There was a $10 DLC pack containing all the digital CE items minus the second copy of the game. Steam took it down about a day after it was put up and Larian said there was a problem with it that they are working to fix and hoping to have it back up as soon as possible. So don't buy anything quite yet, the option to upgrade should hopefully be back up soon.
The $10 DLC has only a few things the DCE has.
Divinity Original Sin - Source Hunter DLC pack:
Unlocks Divinity Original Sin: The Golden Grail DLC, Divinity Original Sin: Zandalor's Trunks DLC, Divinity: Original Sin Design Documents and Art Pack, and Soundtrack. (DLC will only be made available on release.)
Divinity: Original Sin Digital Collectors Edition:
Includes two copies of Divinity: Original Sin, the Source Hunter DLC pack, plus Beyond Divinity and Divine Divinity. Send the second copy to a friend! (DLC will only be made available on release.)
So minus the second copy of D:OS you get 2 other Divinity games for free which are not included in the Source Hunter DLC. I meant he second copy of the game is worth the $30 difference but adding in the DLC and the other 2 games is just a great deal.
I really hope there will be a logical upgrade option from the base game to the DCE. I'd pay the $30 for it and I'm sure others would as well. I really want to continue to support the devs. The $30 upgrade option would allow us to do so.