Kudos to the Aegir project for Aegir's alpha15 release, and especially Miguel for the contribution of a long-anticipated (by me at least) upgrade script. As before, you must read and carefully follow the version-specific upgrade notes at the bottom of the Aegir upgrade instructions, but after that the script automated everything. I was upgrading from the rather antiquated alpha7, and aside from one very minor issue (which was apparently fixed before I posted it to the queue, hehe) everything went smoothly.
Well... almost.
For what it's worth, moving an aegir installation over to a completely new server at the same time as I did the upgrade was... a bit painful. I forgot the aegir_root password. I typed it in wrong - 6 times. I neglected to add aegir to the sudo users. My server didn't verify. My sites didn't open. A few deep breaths and a few restores from backup later, everything works. Now I have my web server running on a nice little mac mini.