Why wait? You can edit your reservation. If you wait until you get to the store it's going to be as-if you had no reservation at all. Just sayin.
As for availability on launch day, I'm sure there will be at least enough where if you got in line by an hour before store opening you'd get one. I'd also expect store employees to either do head counts of people in line to ensure no one in waiting in line for no reason or to announce how many units by model are available.