You will be placed in the bracket that corresponds with your weight at weigh-in, regardless of what weight you entered when you pre-registered.
There is no fee for changing weights.
Pre-registration ends on June 1. On-site registration is available, but there is an additional $10 fee per style. Since there is no payment due at pre-registration, I recommend everyone go this route.
I'll let Will Cokeley respond on how camp participants should pre-register.