Upgraded everything without a hitch. Did aircraft first with remote on and app (android) connected, update percentage was accurate and worked all the way through. Updated slave remote at the same time with an USB stick.
When finished the app said the aircraft would autoreboot but it didn't (good, leaves a chance to look at the log once you've powered it off and before powering up again).
Then updated master and all was good.
Remotes stop once in the middle then continue like last time, don't jump too quickly and turn them off then.
There is a battery upgrade, when the app is connected and the aircraft is powered up with a battery with outdated firmware and firmware file on the SD card it will ask for confirmation then upgrade reboot automatically.