Well, it is booting now...but not without loss of trial options...
After 1 hour, no progress beyond 60%, so I powered it off. I powered it on, and unsurprisingly, it did not boot.
Despite failed firmware update, if the boot loader were okay, then I assumed it may be possible to perform an update via the boot process, if I could get it to work. So (in desperation and with no alternative option!) I tried again to press "HELP" button after power ON during boot. I tried a couple of times, and again no luck, but then it worked!!! I think it is worth stressing that HELP must be pressed immediately after ON button, and well before RIGOL logo appears. It seems that the timing of pressing "HELP" button is very specific, so it may take several attempts to do it right. If you find that the system boots normally despite pressing HELP button, just turn it off and back on, and try again!
This time, update continued with CH1 flashing, and eventually all LED were on. I powered OFF, remove USB flash drive, and power ON. System booted, okay, and System Info says:
Software version: 00.00.01.00.05
Hardware version: 1.0.1.0
FPGA version:
SPU 03.01.02
WPU 00.06.00
CCU 12.29.00
MCU 00.05
So, software version is updated, and hardware version has decreased! (hardware version was 1.1.0.0 before the update).
So, I'm lucky it is working now, but unfortunately I've lost all my trial options. The "Installed" soft-button is grayed out under UTILITY->OPTIONS menu. I hope I can get the trial options back from a new trial license from Rigol.
Also, I'm worried what other system information (calibration, etc.) might have been lost from the failed system update. Does anyone know what are consequences of failed update on system info?
Edit: Emphasis added regarding timing of pressing HELP button.
Edit 2: Improved readability and added more emphasis!