Given NMS has changed so much since it's release, each version of the game plays its own way. I'd instant purchase a 3rd copy on GOG if all previous major versions of the game were still available to download. But I'm betting this isn't the case?

No, there is no previous version of NMS.

I was smart and always downloaded and backed up all versions. I could still play with the versions 1.0.x, 1.1.x, 1.2.x and 1.3.x. That's the advantage of not using Galaxy, downloading everything manually and backing up externally. Same with Steam.

I have every back up of every game I really like. Just in case an update ruins it. BattleTech being one. I have a strange feeling they`re going to screw that with a bad update sooner or later.

That`s the beauty of GOG.