Change USB connection mode while on Fastboot mode I would like to unlock the bootloader of my Huawei P8 that has Android 6.0 and is not yet rooted.

It is shown as connected to the PC when running "adb devices" before rebooting to Fastboot&amp;Rescue mode but after that I cannot change the USB connection type so that it can be connected to the PC in order to unlock it.
You don't need to change the USB connection type of the device due to whenever you boot into fastboot mode your device connection type will be the needed one.

So if your computer not recognize the device in fastboot mode you have driver(s) issue(s).

Try to reinstall your phone's driver and ADB.