Free early checkinn for diamonds.

I see its shown as a perk that if you are a diamond you can change your flight to a early one or a late one same day free of charge.

anybody ever do it? Did you have to ring diamond services?

what does it actually mean free of charge? Does it mean free of change fees but you still have to pay any price difference or is it really free of charge like QANTAS does... you turn up and on checkinn they just reassign you if a seat is free.