Regardless of how you decide to modify the mold base plates, you must create copies of the mold bases before you make modifications to them. Saving modifications to the library assemblies or parts (either generic objects or instances) will permanently alter them.

Do not make modifications to the assemblies in the library. Make modifications only to copies of the assemblies and parts. Set the configuration file options override_store_back to yes and save_objects to all to ensure that the copied models are not saved in their library directory, and that any objects which use the copied models are also saved.