I think, it is depends on not only developer's wish, But on Steam and GoG features. If Larian can remain old D:OS in player's game lists without any inconvenience (and add new game - DOS EE), they'll do. Another way, they can just declare D:OS EE as a major update, but, in fact, almost all files will be changed.
The EE version will appear as a separate entry on user's shelf (at least on GOG). It will be a completely separate product, so there will be no 'rollback' option. Like LordCrash wrote, both could be installed separately.
I was just wondering if there might be such change in EE that some people would prefer the original version instead. And if there are any disadvantages to having both available on shelf even with fresh EE purchase.
