You will, later, get the mileage difference.
Looking at correspondence sent to a HFP reader this week, BA is now claiming that Avios tickets are entirely excluded from EU261 and you will get zilch extra.
This is, even on a casual reading of the rules, bull and EU261 applies but you will need to fight.
However, BA will then claim the 2nd ticket has zero value. EU261 does allow for this. The best you would get via a small claims court is 75% of the one way Avios used for 1 person.
That's why 241 tickets are downgraded first. The new FLY system knows the economic value of each ticket and acts accordingly.