Since many cancel reservations one or two days in advance prior to the penalty deadline you could start looking earlier rather than waiting until the day of. This way you'll avoid any potential hassle with a late cancellation or change.
If you're changing a reservation at the same restaurant on the day of you could explain to the castmembers at the podium and hopefully they or the manager will waive the penalty fee since you are dining there. If you're cancelling and making a reservation at a different restaurant the original restaurant could impose the no show penalty.
Depends how much earlier and what is going on in the restaurant. We've arrived plenty early for our ADR and most times we get seated right away unless it's the most popular restaurant and they are packed. You can always call to see if there is anything available earlier too.