I've just changed my FF number from BA to AY for my next booking. Will see how that works out.
If there are any negatives such as seat changes in the next month, I'll swap back and forth.

Added: noticed from the app that I've now got complimentary messaging for the whole flight(s). Didn't have that on my previous AY flights that had my AY number attached (didn't recognise me on board either)

Additional: seat selection I made as BA Gold has been now removed

Tier points and Avios for return flights on Wednesday posted earlier, about 5 days after the flights.
Earned slightly more even though the fare for the return was cheaper.

Just over a 1/5th of the way to AY silver.
With the flights I'm taking (i.e. LHR-HEL-xxx at around £530 return in business), it works out at around £7.5k for AY Gold / OWS, so no better than actually crediting to BA.