Originally Posted by
FlyingJazzy
..3a: at the gate, ask the gate agent to change the miles crediting from *G to MM silver.
OR
3b: Log in to its MM account or booking management and change the FFP number to the MM silver for miles earning...
Never had an issue to switch to the FTL card at the gate (3a).
3b will in all regularity be declined unless the earning for the *G carrier was zero.