How do I update a virtualenv?

[Skip Montanaro]

I have a virtualenv I'm using for some Django development.  Today I
switched from MacPorts to HomeBrew on my Mac. I'm thus getting a
different version of gcc and its libs. How do I reinstall the
virtualenv? I've looked around and found a few descriptions of what to
do, but as I am little more than a monkey-see-monkey-do user of
virtualenv, I don't understand what appears to be a fairly complex set
of steps. Does virtualenv not have some sort of "upgrade" command that
just does what's necessary?

Thanks,

Skip