I can confirm that V1.2.7 works with boot V1.0.15.
Then, I tried to upgrade using V1.2.8 on page 10.
Havoc did arise: the scope said updating and restarted.
After that, the initial screen was flashing on/off, and the scope wouldn't turn off.
I had to open the scope, remove the battery, wait for some seconds, reinstall V1.2.7 (by using F1+power on, exit F2), and reinstall V1.2.8 (all while on USB and battery removed).
The 703s is working again, phew.