Far be it for me to correct bisonrav whose analysis across these threads have been consistently excellent, but according to BA, it will work as follows:

Your flight on 31st March is awarded in old TP (oTP). It will work as it does now, i.e. based on distance and cabin (+ fare code if economy). It will be applied into your earning year 01/04/24-31/03/25.

Your flight on 1st April will depend on when you booked it. Regardless of when you booked it, you will be awarded new TP (nTP) into your earning year 01/04/25 - 31/03/26

• Booked before 13:00 GMT 30/12/24, take the oTP it would have earned and multiply by 13.3333, rounding up.
• Booked after 13:00 GMT 30/12/14, it will be allocated nTp based upon eligible spend, i.e. cost minus (real) taxes and fees

There appears to be some confusion about the multipliers in use, because in their bid to keep things simple, BA have two different ones:

• Existing bookings made before 13:00 GMT, 30/12/24 have the oTP multiplied by 13.3333 to get nTP
• Existing lifetime TP are multiplied by 15.71 as we switch over on 1/04/25 to give a new balance in nTP