So, Steam has the Digital CE up. $70, includes two copies of Divinity: Original Sin, the Source Hunter DLC pack, plus Beyond Divinity and Divine Divinity.
The Source Hunter DLC pack is available for $10, and it 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. This appears to be the "upgrade" solution for SEA backers.
So, going the "upgrade" path instead of buying a digital or physical CE will result in less stuff, but a lower total cost of $50. There doesn't seem to be a way to upgrade to the $70 digital package.
I'm a little confused by the "Divinity : OS Design Documents and Art Pack", as that is not included in the physical CE.
