Enabling a variant for an already installed port

[Stephen Rasku]

I just enabled bash-completion for MacPorts but the already installed
ports didn't have this enabled.  I would like to upgrade the installed
ports to enable this variant.  Eventually, I would like to do this for
all installed ports but I am starting with Mercurial first.  I tried
this:

    $ port variants mercurial
    mercurial has the variants:
       bash_completion: Completion support for bash
       universal: Build for multiple architectures
       zsh_completion: Install mercurial zsh completion file

    $ sudo port upgrade --enforce-variants mercurial +bash_completion
    --->  Scanning binaries for linking errors
    --->  No broken files found.

    $ port variants mercurial
    mercurial has the variants:
       bash_completion: Completion support for bash
       universal: Build for multiple architectures
       zsh_completion: Install mercurial zsh completion file

It doesn't appear to have worked.  There is no "+" beside
bash_completion and there is no "mercurial" file in
/opt/local/etc/bash_completion.d.

What am I doing wrong?

...Stephen