Modifying mesh of my instance changes prefab also!

I’m currently experimenting with procedural meshes.

My starting block was created with a prefab. When I add more blocks (using the same prefab) and then combine the meshes of these blocks, the Prefab itself is changed by this also.

Is there a way to either modify/replace an instance mesh without changing the mesh of it’s prefab or can I programatically detach an instance from its prefab so I can make modifications without impact on the prefabs?

If my memory serves me right, you can just get a reference to the Mesh, Instantiate it and replace the MeshFilter.mesh or .sharedMesh with the instantiated one in Start/Awake