Link local groups to external files

Hey, I’ve builded a large library with 200+ products for a client. There are several blend files containing the assets and one blend file for rendering with the linked groups. Somewhere on the way the groups in the rendering file became unfortunately local. Is there an easy way to link the local group/meshes to the asset files? I’ve tried to override the group library value with python but it’s read-only. The only other way I can think of, is to link again every group and change the dupli-group everywhere the group is used.