How do I update a virtualenv?

[Ned Deily]

In article 
<CANc-5Uy367Mu-zN30Z8xKfE_Zf9q4G15E_oVtB+HOWPja6MNag at mail.gmail.com>,
 Skip Montanaro <skip at pobox.com> wrote:
> I have a virtualenv I'm using for some Django development.  Today I
> switched from MacPorts to HomeBrew on my Mac.

Any particular reason for doing that?  That seems like a step backwards, 
especially for Python-related work.

-- 
 Ned Deily,
 nad at acm.org