RICKIjustAperson The latest update do include an inbuilt converter, so doing it is very easy.
1 - Download the save
2 - Put the save in your brickadia save folder, usually it is located in: C:\Users\username\AppData\Local\Brickadia\Saved\Builds
3 - Open the game, start a solo play, then press B once you spawned.
Open the "My prefab" tab, you will see a button located on the top right of the window called "Old files"
The game will then open a window of the old .brs files to be converted, you should see the save name. Proceed.
4 - Once done, the window will notify you that everything should be ok.
The save, now coverted as a world, is find in the world folder. Just start a game and select which world to load, the city should be here.
If you have any issues feel free to tell me.
NOTE: With the latest version, for whatever reason, big worlds became unstable, you may experience crashs while loading, if you dont have a big gpu (anything under 8gb of vram will cause issues). I myself have to crash at least once or two times before i am able to load it.