Just hang on the Action object that the `add_argument` returned, and change its `help` attribute.

    a = parser.add_argument('--foo', help='initial help')
    ....
    a.help = 'new help'

If using a custom parser class and subclass, I'd do something like:

    self.changeablearg = self.parser.add_argument...

in the class, and

    self.changeablearg.help = 'new help'

in the subclass

You can test the help message with

    print parser.format_help()