Whoa, I wouldn't volunarily downgrade to a DONE* just for mileage-earning purposes! FYI, in AAdvantage there is little difference between F and J, both earn the same number of elite-qualifying points, but there is a small differential in class-of-service bonus miles (25% in J vs. 50% in F).
The only viable reason to downgrade, IMHO, is because there aren't enough long-haul segments on your routing in F to justify the cost increase. I think you definitely have it covered with lhr-syd, syd-hkg, hkg-nrt-hkg (some flights), hkg-jfk, jfk-sfo (some flights), lax-lhr, and lhr-dxb-lhr, and possibly some of the S.America routes.