1. Open the base animal's .nif and delete the NiBoneLODController.
2. Now remove all NiNodes that contain 'LOD' and/or the base animals name..
3. Open your animal's nif. Right-click on the model and select Block-Copy Branch.
4. In the base animal's nif right-click on Scene Root and select Block-Paste Branch
5. Save the .nif