I am a trustee for my State's Boy's State and am the chairman for the 21 Legion posts in my county. Here, if you are an alternate, it is because the post already has its slots filled and you are on a waiting list. In this county, if a boy cancels, the post pushes one of their alternates into the slot as you would expect but if all of their alternates are "used" then they turn their open slot to me and I roll alternates from elsewhere in the county. Here in NJ, alternates who are still alternates when Boy's State starts (Father's Day) are encouraged to show up ready to go and they are moved in to fill vacancies that invariably happen. For the last 10 yrs or so, everyone who showed up as an alternate ended up staying for the week.