It doubt it will make much difference but there will be a slight drop in density - BA get 56 CW seats into the same space CX could get ~48 Cirrus. Yield dropped a little in recent years so i'd expect less sale fares and aggressive corporate discounts once the seat is fully rolled out. I doubt the average passenger would notice much difference.
After all, if BA didn't think a new seat would have a positive impact on yield they wouldn't be doing it.