I don't see really what changes, I paid full freight for the original surely that's worth something. I dare say, the original is probably now going to be abandoned by the dev in it's current buggy state.
I got the newsletter telling me about the discount for owners about a week go.
It was a preorder offer, the same offer was given to those players who own the game on EpicGames.
It's a decision made by the publisher, not GOG.
The original was abandoned a long time ago, the new rights owner never released any patches. There is a unofficial fan patch however. https://www.moddb.com/games/xiii/downloads/unofficial-xiii-patch-15
Err...That's for XIII. Different game.