change_obj_and_mat_IDs.py

import bpy

OLD_DEFAULT_ID = -1
NEW_DEFAULT_ID = 0

# Object IDs
for obj in bpy.data.objects:
    if obj.luxcore.id == OLD_DEFAULT_ID:
        obj.luxcore.id = NEW_DEFAULT_ID
        print("Changed ID of object", obj.name)
        
# Material IDs
for node_tree in bpy.data.node_groups:
    for node in node_tree.nodes:
        if node.bl_idname == "LuxCoreNodeMatOutput" and node.id == OLD_DEFAULT_ID:
            node.id = NEW_DEFAULT_ID
            print("Changed ID of material node tree", node_tree.name)

print("Done.")