I would sit tight and don’t accept the downgrade for now.
On the days I’ve looked at on ExpertFlyer BA5/6 is being operated by a 787-9 and BA7/8 by a 4-class 777.
However on both flights the First booking classes are not on sale and the First cabin is shown as part of Club World with all the seats blocked from selection.
I strongly suspect this won’t be the arrangement that actually flies most days of the Winter season and there will be further changes/updates.
If you were on a cash ticket I might have suggested having a go at getting moved onto JAL. I think the one report we have had on that was that it wasn’t successful despite the Joint Venture and the BA code shares on the JAL flight, but YMMV. However I think virtually zero chance of this on a redemption.