How do I update a site after its installation profile is updated? Say I decide to add a new module to an installation profile I've created and used to build a bunch of sites with.  Can I easily use Drush to update all of those sites to the latest version of the Installation Profile?

I've just recently started to grok installation profiles, but I'm not sure if they are useful after a site has been set up.  I manage 40+ sites at work, so this would be amazing if possible.

TIA.

Aaron
I don't know if you have any problem using Drush to update the installation profile, but I use git and everything is fine.

If you have code (in the installation profile) that writes on the db and push this to production nothing will happen. So all your changes must done manually.

Finally, the installation profiles aren't the best way to maintain a site, after Drupal install nothing will happen.

About the 40sites take a look at Aegir. It's a hosting platform for Drupals.