Originally Posted by
corporate-wage-slave
Old system = old results, no change. So the answer is end October 2025. There is a definitive answer, it has been mentioned since page 2 of this thread and repeated fulsomely. In your case those 300 TPs could help you get to end April 2026 since you have quite an overlap, but you nevertheless would need 1200 further TPs between April 2024 and 31 March 2025, so the first set of 1200 TPs are unfortunately in the wrong place for that year (starting 1 April 2025).
There isn't a definitive answer from BA though. The only clear answer is with respect to 'existing' status which this isn't, yet. Indeed you'll see in my post here
https://www.flyertalk.com/forum/35973086-post390.html that HeadforPoints have got this wrong despite being in touch directly with BA. They think the answer is April 26.
I've reached the same conclusion as you, but that's not thanks to any definitive answer from BA on the matter.