Was playing around with key sequences during power up with my Rigol DS1074Z-S (converted to a DS1104Z of course) and discovered how to force a firmware downgrade. So far I have gone back and forth between a few versions, the oldest being to 4.00.00.00. No side effects of any kind were noted so far. I am sure this work for the entire DS1000Z line including the ever popular DS1054Z.
If you follow my instructions you do so at your own risk, if your scope bricks or steals your TV remote I am not responsible!
*edit* - this appears to work only with very old boot loader versions such as 0.0.0.11 and 0.0.0.13
To downgrade do the following:
-Copy the .GEL file with the firmware version you want to a USB stick that you know is compatible with the DS1000Z series.
-With the scope powered OFF plug the USB stick into the scope.
-Power the scope on.
-When the splash screen with "Rigol" appears you should hear a quick beep which indicates it sees a USB device.
-As soon as you hear the beep press the "Help" button 3 times quickly.
-In a few seconds you should see the channel 1 LED blinking - this indicates it is flashing the firmware.
-Once done flashing the scope will boot as normal and tell you it found a firmware update with the same version it just flashed, you can press cancel.
-You can then verify the new detailed version info by pressing "menu menu force menu" quickly, then pressing "utility","system", "system info".
-I'd reboot at this point before using the scope just for good measure.
Being able to downgrade the DS1000Z series of scopes doesn't seem critically important at this point but there does seem to be a laggy version people may want to downgrade from.
For me the bigger picture is pondering what other devices this method might possibly work for - my mind instantly goes to the DSA815 spectrum analyzer. There is a slim chance those with the newest bootloader version could use this method to downgrade firmware so that Riglol works again, install all of the licensed options and then upgrade as normal to current firmware while retaining the options. I don't have a DSA815 anymore to try it, if someone does attempt I expect they will have sweaty palms.
Attached are before and after system info screen shots from my scope.