Crediting flights to Qantas frequent flyer - BA silver member

I’m flying a lot with a mix of BA and Qantas over the next 6 months. I am a current BA silver. It’s pointless crediting all my flights to the BA scheme, I’ll only get to bronze, which I will be anyway next year due to dropping form silver.

I have worked out that if I credit all flights to the Qantas scheme, I should reach their gold tier which is the equivalent of BA silver.

I’ve already taken one European flight, that has been credited to the BA club, can I retrospectively change this over to the Qantas programme?

Also how do I ensure that all future flights are credited to the Qantas scheme, not the BA scheme, but ensuring I am able to keep all my silver benefits (free seats etc)