You won't be able to buy the original version after the EE is out.
Was that stated somewhere?
I recall Swen saying that but I sadly don't have a concrete link for it right now. But it's definitely possible that I mixed somthing up and what I said is wrong...
I'm pretty sure, Swen answer this question every Larian's stream wich includes latest marathon stream. And I also recall, He said that old version won't be available.
By the way, GoG users can play both version due to possibility to cancel update. Due to DRM-free system, They can just save old version in one folder and Enhanced Edition in another.
UPD I've found this statement in Reddit AMA (click to increase). May be I've misunderstood.
DOS EE won't be an update to the existing DOS anyway. It will be a completely different game in both libraries both on Steam and on GOG. So everybody who owns DOS already will be able to play the original version no matter what. Everybody can have both versions installed simultanously. The only kind of option question is whether Larian will still sell the old version. Personally I doubt it because that would only irritate new people who might buy the old version, not knowing that there is a newer version of the game available. I mean, there is no purpose in still selling the old version if Larian thinks that the EE is the better product.