Before you book your flight, check if award space for first is avaiable like in the pic.
If it show "2× Avios", then you cant upgrade from biz to first.
Because i was flexible regarding my flight time/day i just searched a bit to find a day on which first award space was available same on the pic above. Booked in J and right after booking upgraded online to first with my Qcredits which was confirmed instantly
Now i am on 01A :-)