Yes, once loaded in the scene you can modify any item without modifying the library.
All models are stored in the layer "forest_templates", which is hidden and freezed. So, the first step should be to show and unfreeze this layer.
Next, each model is assigned in the Geometry rollout with an unique name. To select them easily, use the quad menu: right click->Select Custom Object.
Please note that models may be loaded directly in the scene or as XRefs, depending the mode you selected in the Library Browser.