A quick update on my experience: when I spoke with Jeppesen on the phone, they apparently put in the wrong serial number for my G-600 (I'm pretty sure they put in the serial number for my GTN750 for the 600, as well as the 750). So the 650 and 750 updated just fine (as did the GTN 625 in my Citabria), but the G-600 complained about the update failing.
Another call to Jeppesen, they figured out that the serial number for the G-600 was wrong, wait a few days, and now, today, everything downloaded and updated just fine. So all is good!