My question about the changes is why doesn't BA announce all the changes at once, with full details of all changes, rather than partial information spread over a period of two months?

I can only think of these reasons

a) BA didn't decide on all the changes so released them as they decided. I cannot believe a major corporation would work like this.

b) BA wanted to release some (good) news first, then mix in some bad news behind that.

c) Depending on the reaction to the first wave of announcements, the complete changes to the program are modified (eg more TP for long haul should the first announcement of avios be very badly received).