convert_dictionary.py

#!/usr/bin/env python3

import sys

from plover.dictionary.base import create_dictionary, load_dictionary

input = sys.argv[1]
output = sys.argv[2]

id = load_dictionary(input)

od = create_dictionary(output)
od.update(id)
od.save()

I needed to add the following:

from plover.registry import registry

registry.update()
system.setup("English Stenotype")

Otherwise it could crash with the following error:

AttributeError: module 'plover.system' has no attribute 'IMPLICIT_HYPHENS' 

Also, in a sufficiently new Plover version, you can right click at a dictionary and choose "Save dictionaries as..." to convert between supported types.