Apply changes to an Asset (after changing its mesh)

I'd like to add UV2 to the mesh of imported asset ('planet_spherical_20kVerts', as seen on the image).

Spoiler: Code

Code (CSharp):

1.  
2.     //assigns a custom UV2 coordinate to every vertex of every child mesh.
3.     void bake(){
4.         if(_myAsset == null){ return; }
5.  
6.         List<MeshFilter> mfs = _myAsset.GetComponentsInChildren<MeshFilter>().ToList();
7.         foreach( var mf in mfs){
8.             Undo.RegisterFullObjectHierarchyUndo(mf, "undo mesh filter bake");
9.             Vector3[] verts = mf.sharedMesh.vertices;
10.             int len = verts.Length;
11.             Vector4[] attribs = new Vector4[len];
12.             System.Diagnostics.Debug.Assert(len % 3 == 0);
13.  
14.             for(int i=0; i<len; i+=3){
15.                 attribs[i+0] = new Vector4(1,0,0,1);
16.                 attribs[i+1] = new Vector4(0,1,0,1);
17.                 attribs[i+2] = new Vector4(0,0,1,1);
18.             }
19.  
20.             mf.sharedMesh.SetUVs(1, (Vector4[])attribs);
21.             EditorUtility.SetDirty(mf);
22.             EditorUtility.SetDirty(mf.sharedMesh);
23.             EditorUtility.SetDirty(_myAsset);
24.         }
25.  
26.         AssetDatabase.Refresh();
27.         AssetDatabase.SaveAssets();
28.     }//end()
29. }
30.  

This correctly updates the shared meshes of this asset. However, the changes revert if I right-click and re-import this asset. The same happens if I exit the editor, and come back - the asset has the 'old' mesh.

How can I "bake-in" these changes to the shared mesh of an asset, so this asset never reverts them?