BA's FAQ suggests to me that it'll stay under the old scheme, with the conversion rate. It's not 100% clear, but the latter paragraphs suggest existing bookings can be changed, but still retain that feature?
How are my Tier Points impacted if I change a booking for travel after 1 April 2025 and the new price is higher or lower?
From 1 April 2025, you’ll earn Tier Points based on total eligible spend. You’ll also earn Tier Points for any qualifying add-ons purchased as part of your trip. If you make any changes to your trip and your ticket price changes, the Tier Points you earn will be based on the total eligible spend for your updated ticket.
For flights booked before 13:00 GMT 30 December 2024 for travel from 1 April 2025, that are marketed by British Airways, American Airlines or Iberia, Tier Points will be awarded based on a conversion of the existing method. This means any bookings you’ve already made will earn proportionally the same number of Tier Points, or more, as today