Gaia keeps delete my terrain

[Pac]

I use gaia to generate terrain and edit it then save it into prefab for later use, but when I start generate a new terrain, it keeps deleting my previous terrain which is already in a prefab, even the terraindata became unusable, tried to export heightmap and creat a new terrain then import and edit, but when I generate a new terrain using World Designer, it even delete the terrain that I just import the exist heightmap and edited

[Bryan]

That's correct there is a pop-up that states any existing terrain will be destroyed. 
A way to stop this from happening is please disable any terrains you have active. 
Then when you create a new terrain after its completed you can re-enable the terrain again.