Recreated worlds when placed in "saves" folder

When I put an old world in my "saves" folder in .minecraft, it is recreated and it loses every change I have done.

What I expected to happen was to continue my old worlds but with a new version of Minecraft.

What actually happened was that I lost all progress made in the world.

Steps to reproduce:
1. Make a world with any version of Minecraft
2. Update Minecraft
3. Put your old world folder in the new "saves" folder
4. Open Minecraft, go to that world and it will be recreated