import hou


def replace_type_instances(src_type, dst_type):
    for instance in src_type.instances():
        if instance.isEditableInsideLockedHDA():
            keep_network_contents = not instance.matchesCurrentDefinition()
            new_node = instance.changeNodeType(
                dst_type.name(),
                keep_parms=True,
                keep_network_contents=keep_network_contents
            )


def get_hda_first_type(hda_filepath):
    definitions = hou.hda.definitionsInFile(hda_filepath)
    for definition in definitions:
        return definition.nodeType()


def main():
    src_type = get_hda_first_type('/path/to/src.hda')
    dst_type = get_hda_first_type('/path/to/dst.hda')
    if src_type and dst_type:
        dst_type.definition().setIsPreferred(True)
        with hou.undos.group('Replace type instances'):
            replace_type_instances(src_type, dst_type)


main()