Note that the online transfer just cares about the Aeroplan number. I successfully moved ~30K of my dad's Amex points into my Aeroplan account.
However, you can only have one Aeroplan number registered with the Amex online transfer tool -- I don't know how my dad would go about getting that changed if he wanted to then transfer to his account. In our case it didn't really matter as he closed the account to set up for the next churn.