If I have a Mileage Saver award ticket for later in the day, but I show up for an earlier flight (same segment, of course) that has empty seats, will I easily get a seat -- even if the available award seats for the earlier flight were not Milage Savers?

I'd imagine that the answer is "yes" (though it may not be official policy), but I'd like to confirm.

But how about the reverse? That is, what if I book a Mileage Saver ticket for earlier in the day, but want to fly on a flight later that day? Can that be done without my res being completely cancelled as a no-show?

Thanks.