GOG removed "Mark of the Ninja" from its catalogue, the day of the addition of "Mark of the Ninja: Remastered"
Why?
https://www.gogdb.org/product/1207665123#downloads

We’re not allowed to buy old-fashioned games here anymore?
It's not the same game !
https://www.pcgamingwiki.com/wiki/Mark_of_the_Ninja

The first game is very good, and GOG has the Linux version!
(gog_mark_of_the_ninja_2.0.0.4.sh) Which works very well on 64-Bit Os!

Yes, I have noticed that today. I own the old one, and it no longer lists in the store. That brings me to another question - would I my version get swapped with the new one? How does this work?

It depends on the studios,
In our case, Klei Entertainment sells the Remastered as a new game.
We can understand, a job worth wages...

But the disappearance of the original, means more updates.
The risk; one day the game will no longer be compatible for any reason, no passing 64Bit etc...
This game will die and Klei will earn more money selling a Remastered as a new game, to people who have already bought the first abandoned version.
( 1 game, 2 sales )