If you build the brackets prior to weigh-in they will change. They will need to be built again. It's just more work.
Once the seeding meeting is done the brackets should never change even if a kid doesnt show up or doesn't make weight. If that happens then it is a scratch in the bracket and the person that they were to wrestle gets a bye. That's how it has always been when it comes to Subs. The only case I can see where it may completely change a bracket is if 6 kids in an 8 man bracket and 3 of the kids don't make weight leaving 1 kid with a bye to the finals. In that case the bracket needs rebuilt to be a 3 man round robin. (there would be other cases too but not that many instances)
