Understand the loader question before installing any Lost Cities package labelled for Fabric. This guide focuses on a repeatable setup process rather than assuming every launcher, modpack or server uses the same files.
Know the main project baseline
The modern main Lost Cities project is documented around Forge. That means a Fabric-labelled package should not be assumed to be the same official build.
Verify the source
Check who maintains the Fabric package, which Minecraft version it targets and whether it requires extra compatibility libraries.
Do not mix loader files
Forge and Fabric mods use different loader ecosystems. Keep separate launcher instances so files and dependencies do not get mixed accidentally.
Test with a new world
Even when a third-party port launches successfully, generation behavior may differ. Use a test world before moving a long-term save to a different loader setup.