I’ve had considerable difficulty switching FF numbers midstream. In my case, I hold status in both, but sometimes did not know at booking which program I wanted to credit, and it was also a lot quicker to book on the BA website using my BAEC number.
i did always manage to get the flights credited as I wished after flying, but it was a headache. And I did encounter some verbiage from both BA and AA about their first checking that I had not received any benefit from using the program number from which I was trying to switch. So while your mileage may vary (as we say here), just be prepared that it may not work out as you wish.