I personally didn't turn off the high voltage battery, but you really should if you follow the BMW guides. my reasoning for not - the headhunt is 12v so I wasn't too worried and it didn't cause any issues.
the only other thing I would add. I didn't unplug all the cables from the back of the gear stick surround for things like reverse camera button etc and dashboard lights. I should have just spent the extra 1 minute unplugging them all as I spent an extra 30 mins trying to work with the lack of space whilst keeping them in place.
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