def METHOD_NAME(bl_id):
    """Register old node class"""
    if bl_id in old_bl_idnames:
        mod = importlib.import_module(".{}".format(old_bl_idnames[bl_id]), __name__)
        res = inspect.getmembers(mod)
        for name, cls in res:
            if inspect.isclass(cls):
                if issubclass(cls, bpy.types.Node) and cls.bl_idname == bl_id:
                    if bl_id not in imported_mods:
                        mod.register()
                        imported_mods[bl_id] = mod
    else:
        raise LookupError(f"Cannot find {bl_id} among old nodes")