Your compensation would be the difference in avios between F and J for that sector. As it's more than 14 days before departure BA can theoretically cancel your whole booking without paying you and compensation beyond a refund.
This is BS!!!
The 14 day period only apply to article 7 compensation. Article 7 usually does not come into play in cases of downgrade.
A downgrade triggers the refund of 75% of the total costs of the ticket segment. Hence, if LHR-ORD costs 100k points, then BA has to refund 75k + 75% of all fees/taxes.