I did this a few times, out of necessity ("near-brick
experiences"). I think I used these instructions:
http://wiki.laptop.org/go/Activated_Upgrade
No developer key involved, but I should probably get one
anyway. Where is that info?
Other upgrade methods are better if you are not starting with
a near-brick and want to retain some data:
Developer key info is at
http://wiki.laptop.org/go/Activation_and_Developer_Keys .
I was able to upgrade the olpc-update command and download build 653
without it, but I was unable to *complete* the installation and boot the
new build without a developer key. I am told that others did not have this
experience. It's possibly a timing issue -- that when I got build 653, it
had not been marked as a non-developer upgrade yet, and that since then it
has been; all I know is that I was unable to complete the process without
the key. (This was around Jan 1.)