You have to go to the airport on the day and ask to switch to an earlier flight. If there’s a seat, it’s yours at no extra cost.I usually request it in the lounge or at the gate. The gate can add you to the flight up until they close boarding, so go right there if there’s a flight about to leave.

If you call the call centre normal procedure is to charge you the fare difference, so this really is a benefit to use at the airport only.

If there is no aerspace seat available, you can sit in a regular seat with no free food or drink, but no refund as you asked to switch. You’ll still get aerspace tier points though