I went over Techflaws update.log with a fine toothed comb… The only error with upgrade vs downgrade in the log was…
Upgrade
[apache-php-webdav.preinst] 03/02/13 11:29:58: update 02.00.01-88851
Remove all non-device users
Delete apache user pwd for user=admin id=1
- deleting apache user admin in /etc/nas/apache2/auth/htpasswd
Deleting password for user admin
Downgrade
[apache-php-webdav.preinst] 03/27/13 14:18:36: update 02.00.01-88851
Remove all non-device users
Delete apache user pwd for user=admin id=1
- deleting apache user admin in /etc/nas/apache2/auth/htpasswd
User admin not found
No other errors present
Also I believe Techflaws used updateFirmwareFromFile.sh instead of the dashboard to do the downgrade
updateFirmwareFromFile.sh:03/27/13 14:14:23:upgrade start: /CacheVolume/DOWNGRADE.ap2nc-024006-048-20121206.deb
updateFirmwareFromFile.sh:03/27/13 14:14:23:check_size=
updateFirmwareFromFile.sh:03/27/13 14:14:23:version_current=024105034
updateFirmwareFromFile.sh:03/27/13 14:14:23:version_newfile=044006048
updateFirmwareFromFile.sh:03/27/13 14:14:23:package_newfile=ap2nc
updateFirmwareFromFile.sh:03/27/13 14:14:23:master_package_name=ap2nc
updateFirmwareFromFile.sh:03/27/13 14:14:23:master_package_new=ap2nc
updateFirmwareFromFile.sh:03/27/13 14:14:23:update_container=
(Reading database ...
So far everything is running fine for him, but there is always the possibility you could run into issues later on down the line with an upgrade as Bill_S and Myron have stated. But even then you could fix the drive by pulling disk A and running a debrick script from my guide.
And as always have a backup of a backup 
http://community.wdc.com/t5/My-Book-Live-Duo/GUIDE-Debrick-a-MyBookLive-DUO/td-p/544860
