Hello LKSI,
There is way to force the ecu in bootloader mode called serial loopback, i'll outline the procedure for you:
1) power down the controller (v3 ECU or round)
2) remove fuelpump, injector and ignition fuses (or relay), and remove injector connectors.
3) disconnect any device (usually PC) from the controller (GenBoard or round)
4) connect the ECU RS232 (serial port) DSUB9 pin2 to pin3, with a wire, a conducting paperclip, or a dedicated loopback connector
5) power up v3
6) remove the loopback
7) connect PC to v3 RS232 (serial port)
8 ) start VT, in preferences->communication settings press the detect button, it should detect the ecu
9) in the popup which says bootloader mode detected press the stay button
10) in tools firmware->firmware upload _do not use the wizard_ select your previous firmware and upload it to the ecu.
11) upload a know good config to the ecu (saved map from before upgrade gone bad).
After this your ecu should be behaving as before the firmware upgrade, if you run into any trouble please report.
Best regards, Dave