Got an answer to my question from AC, so posting here in case anyone else is wondering. Short answer: After selecting bonus miles privilege accounts will be credited based on activity to date.

Transcript:

Hello ,

How may we assist you today?

Best regards,

Air Canada Social Media /cp

ME

I'm wondering how the new, extended timing for choosing Altitude Select Privileges will affect bonus miles option: If I wait until September to choose bonus miles, will I get bonus miles credit only for miles accumulated after I make the selection?

Hello ,

The selection would be effective on flights as of January 1st.

Regards,

Air Canada Social Media /cc

ME

Great. Thanks for clarifying. Can you also tell me when the bonus miles are actually applied to my account?

Hello , The bonus miles should be credited to your account within a few days after each flight is flown. Regards, Air Canada Social Media /cc

ME

So just to make sure I understand: If I select 50% bonus miles privilege on September 1st, I would immediately receive a credit to my account based on all activity Jan - Aug?

Hello , Just to clarify, likely not "immediately" but not long after. Regards, Air Canada Social Media /cc

ME

Perfect. Thanks for your help