Firmware upgrade is sequence of two steps. Download firmware, flash firmware. If you phone is still in "download phase", then you can powercycle them - old firmware is stil untouched. No reason to hesitate.
But if it stuck in second phase, there is no valid firmware in the phone. You can do nothing with it as long as phone is bricked. You can do power cycle only. Phone will start on S.O.S. Recovery mode after it.
Download firmware recovery utility with 7.4.9c firmware (from same place as standard firmware image for your device) and run them on Windows computer connected to the same network as your phone is. Select "Recover Firmware", write serial number of affected phone and select YOUR ip address from combobox.
It will recover your phone to 7.4.9c version.
Don't load newer firmware recovery utility. Most of phones has 7.4.4rec bootstrap image inside and it can't be recovered to post-7.5.2b firmwares directly. Use an pre-7.5.2b recovery utility then upgrade the phone standard way.
