bbHTTP2HTTPS plugin by Bjorn Wijers that updates all active(!) WordPress Mu blogs in one installation to use https instead http.
It does NOT change the guid or post content with regards to the links used in there.
The author says, you might still have to change some links in your content after switching from http to https. As always, first make a backup of your WordPress Mu install before using this plugin. This plugin might working in previous versions of WordPress Mu as well, but this has not been tested.
Download and info: http://wordpress.org/extend/plugins/bbhttp2https/
