In addition to BPry's comments (completely accurate), to help anyone running in the same issues after the looong M-600 Panorama upgrade, the M-600 did not show any more firewall logs after the reboot. Neither real-time nor pre-existing logs were being reported. After an hour-long remote support session with Palo's support, they too were at a loss as to the root cause. They collected Panorama's logs to review them offline.
The morning after (about 7 hrs later) I checked again just in case... and Panorama was now showing the real-time traffic and had a couple of months worth of past logs. It took the rest of the day, but by the evening all historical logs had reappeared as well.
So don't panic if you don't have data after the v9 - v10 upgrade for several hours. They will eventually appear.