Will I earn fewer Tier Points on my existing bookings?
No. Any bookings made on or before 27 April 2015 will be honoured under the current rules which means that you will still earn 50% of the Tier Points that would be earned in flexible economy for bookings made in any lowest economy fare class. Only new bookings made on or after 28 April 2015 in our lowest economy Q, O and G classes will earn 25% of the Tier Points.
https://www.britishairways.com/en-gb...aqs#tierpoints
Seems to imply that you would retain the TPs and avios under teh existing earning rates as long as the booking was made prior to 28 April 2015. If the answer were that you get the new rates would your booking strategy be any different though?