For info
When formating the USB make sure it its FAT32 and Cluster size to 4096 bytes (some of my newer USB's has 8192 as default).
I updated to 01.03.00.03 and I can confirm the SSH GEL still worked (but I don't remember where I downloaded the .03 firmware, as I don't find it on Rigols hompeage anymore).
When trying to downgrade using the "secret" menu, pressing SINGLE button twice at boot, I got "Failed to upgrade! Check the upgrade file."
My solution was to use another USB, even tho I knew the first USB worked for upgrade, apparently it didn't work for downgrade.
Hope this helps someone!