I think I've got the hang of this...

I don't fly enough for *G, but *S and its free bags may be nice.

My understanding is I sign up for A3 and fly 3000 miles (for a total of 4000 miles, and assuming mileage-earning booking classes and so on) within 1 year.

I am now *S for 12 months from the date I hit the 4000 miles. These miles now expire in 1 year, and I can retain *S if the number of miles in my account never dips below 4000 due to expiry.

Now, I don't want to credit any more miles to A3, so I can put down any FF number on future tickets and just show my *S card while checking in to get bag fees waived.

Is that correct?