So it is BA53 on 19 May.
It helps because we all can see the seat map and load on EF.
Apparently thisflight is very full. It could be that you have been targetted as potential opup given your status and fare. But there is nothing definitive yet, so you will have to wait for OLCI or at airport.
I would assume that your seat 15K is still yours but that they plan to opup you to 2F. Currently all F seats are assigned on EF. And calling for a seat change of a potential opup is going a bit overboard.

But it could also be a glitch, a potential ac swap or any other.
In any case, I would relax and wait for OLCI with a potential good news.