Cannot downgrade from 1.14 format to 1.6 or 1.9 format

I just fetched 1.14 to try on MacOS X to mess-about with the EOL support.

My first test was to upgrade a small repo from a 0.92 to the new 1.14 format, which succeeded and took very little time.

On a whim, I tried to downgrade to 1.9, but this process hangs.

$ tbzr upgrade --1.6
starting upgrade of file:///private/tmp/dotfiles/
making backup of file:///private/tmp/dotfiles/.bzr
  to file:///private/tmp/dotfiles/backup.bzr
starting repository conversion
repository converted
[###################/] checking repository format 1/1

and the spinner just continues to spin forever (well, at least 5 minutes).

I can upgrade and downgrade from 1.9 to 1.6, but not from 1.14 to either 1.6 or 1.9.

I've tried sampling the stack by breaking and looking at the trace with pdb (about 10 times over 3 minutes): the process does seem to be doing something, but several of those traces involved options and config objects, like the following:

Pdb) bt
  /scratch/dev/bzr-1.14/bzr(130)<module>()
-> exit_val = bzrlib.commands.main(sys.argv)
  /scratch/dev/bzr-1.14/bzrlib/commands.py(968)main()
-> ret = run_bzr_catch_errors(argv)
  /scratch/dev/bzr-1.14/bzrlib/commands.py(979)run_bzr_catch_errors()
-> return exception_to_return_code(run_bzr, argv)
  /scratch/dev/bzr-1.14/bzrlib/commands.py(727)exception_to_return_code()
-> return the_callable(*args, **kwargs)
  /scratch/dev/bzr-1.14/bzrlib/commands.py(922)run_bzr()
-> ret = run(*run_argv)
  /scratch/dev/bzr-1.14/bzrlib/commands.py(559)run_argv_aliases()
-> return self.run(**all_cmd_args)
  /scratch/dev/bzr-1.14/bzrlib/builtins.py(2969)run()
-> upgrade(url, format)
  /scratch/dev/bzr-1.14/bzrlib/upgrade.py(85)upgrade()
-> Convert(url, format)
  /scratch/dev/bzr-1.14/bzrlib/upgrade.py(40)__init__()
-> self.convert()
  /scratch/dev/bzr-1.14/bzrlib/upgrade.py(79)convert()
-> self.bzrdir = converter.convert(self.bzrdir, self.pb)
  /scratch/dev/bzr-1.14/bzrlib/bzrdir.py(2714)convert()
-> branch = self.bzrdir.open_branch()
  /scratch/dev/bzr-1.14/bzrlib/bzrdir.py(1616)open_branch()
-> return format.open(self, _found=True, ignore_fallbacks=ignore_fallbacks)
  /scratch/dev/bzr-1.14/bzrlib/branch.py(1604)open()
-> ignore_fallbacks=ignore_fallbacks)
  /scratch/dev/bzr-1.14/bzrlib/branch.py(2373)__init__()
-> super(BzrBranch7, self).__init__(*args, **kwargs)
  /scratch/dev/bzr-1.14/bzrlib/branch.py(1882)__init__()
-> Branch.__init__(self)
  /scratch/dev/bzr-1.14/bzrlib/branch.py(94)__init__()
-> self._open_hook()
  /scratch/dev/bzr-1.14/bzrlib/branch.py(2357)_open_hook()
-> url = self.get_stacked_on_url()
  /scratch/dev/bzr-1.14/bzrlib/branch.py(2538)get_stacked_on_url()
-> stacked_url = self._get_config_location('stacked_on_location')
  /scratch/dev/bzr-1.14/bzrlib/branch.py(868)_get_config_location()
-> location = config.get_user_option(name)
  /scratch/dev/bzr-1.14/bzrlib/config.py(175)get_user_option()
-> return self._get_user_option(option_name)
  /scratch/dev/bzr-1.14/bzrlib/config.py(684)_get_user_option()
-> value = source()._get_user_option(option_name)
  /scratch/dev/bzr-1.14/bzrlib/config.py(353)_get_user_option()
-> for (section, extra_path) in self._get_matching_sections():
  /scratch/dev/bzr-1.14/bzrlib/config.py(497)_get_matching_sections()
-> sections = self._get_parser()
  /scratch/dev/bzr-1.14/bzrlib/config.py(310)_get_parser()
-> self._parser = ConfigObj(input, encoding='utf-8')
  /scratch/dev/bzr-1.14/bzrlib/config.py(143)ConfigObj()
-> return _ConfigObj(*args, **kwargs)
  /scratch/dev/bzr-1.14/bzrlib/util/configobj/configobj.py(1276)__init__()
-> self._load(infile, configspec)
  /scratch/dev/bzr-1.14/bzrlib/util/configobj/configobj.py(1345)_load()
-> self._parse(infile)
  /scratch/dev/bzr-1.14/bzrlib/util/configobj/configobj.py(1704)_parse()
-> this_section.inline_comments[key] = comment
> /scratch/dev/bzr-1.14/bzrlib/breakin.py(33)_debug()
-> signal.signal(signal.SIGQUIT, _debug)
(Pdb)