Hi, i'm afraid it cannot be done automatically, but with some manual work this would be the process:
All library models are stored in the "forest_templates" layer.
1) Select the layer and delete all "forest_library_???" objects (but not the "forest_template_???" ones).
2) For each one of the Forest objects, select each one of the items of the Geometry List and assign it from the library again, using the XRef option.
That will keep the existing distributions, but replacing all models. I hope that helps.