Thank you for the comments so far. I have checked and the flight has been ticketed.
It does seem that downgrading could happen on the 75% rule if they chose but it would be very disagreeable particularly since they were still prepared to sell at least one J ticket as early as yesterday, but not now. Overbooking and then downgrading the Avios passenger would be shameful but hopefully it won't come to that.
I will update tomorrow to close out.