Follow-up question:

My parents have a trip to Lima on UA coming up and will make about 6K miles, and at 50% credit to A3, they'll end up with Star Silver (due to the 1K signup bonus). In principle, they will now get a free back for transborder flights.

Now, until they get the Aegean Blue (i.e Star Silver) card, they MUST have their A3 number on the BPs and credit the flights to A3 and not to Aeroplan. When the A3 card comes, they can credit to Aeroplan and just show the A3 card at check-in to get the fee waived. Is that correct?

Basically I think "losing" the 6K miles credit to Aeroplan is worth it for them to get Star Silver on A3 (for the free bag), but not if they now have to credit to Aegean every time they fly...