While it is frustrating when you aim for a particular a/c and there's a swap, we're talking about flights respectively 7m and 9m in the future right? Therefore I don't think BA are being particularly unreasonable in managing their fleet resources in this way. And at least you have ample time for notification.

I've had same day swaps and that's a bit more frustrating - looking at it on a phone screen en route to the airport.

And you two sound like somewhat frequent flyers, so presumably aware that this sort of thing is always a possibility.

Annoying but that's about it really. I personally would concentrate on getting the best seats you can on the new metal but that's just my opinion and doubtless others will differ (and for the record I'm no huge fan of J in the triple either, although Iam not sure I see where OP is going on the seat issue - I find them to be the same as A380 / B744 J seats myself. Just cabin layout differs)