Thanks for the suggestion to restore from the config backup. :)
I wasn't aware I could selectively restore stuff, but then chose to restore only the rrd data. I was logged in through ssh during the whole process and it was actually fun to watch it upload the data, incorporate it into config.xml, then write out the /var/db/rrd, and remove it from the config file. Before rebooting, it saved the /var/db/rrd data into rrd.tgz, and unpacked it again after the reboot. So, my data is back, with only a short missing piece during the upgrade process.
Roman