Prefabs "resetting" when hitting Apply

I'm making my NGUI UI out of various prefabbed panels. I have always just duplicated a previous panel and used that to start on the next one. Now however, I have to create new panels, sprites, and buttons from scratch because of an odd prefab issue. If I duplicate prefab-1 and rename it prefab-2, then make various changes that break the prefab connection in the process. Then I drag prefab-2 into my Resources folder to ensure it becomes a "new" prefab, right? Well both that drag and drop, and any subsequent clicks on the Apply button for that prefab will reset any sprite associations on the child GameObjects. Their sizing and positioning remains, just the sprite graphic itself reverts to what it was on prefab-1. Any ideas?