My understanding is that you can modify as long as the LL has not expired. We always modified 10-15 minutes before expiration at the latest. I was not brave enough to see what happens when the LL hits the buffer (I.e., the grace period).So, at what point does your LL lock in where you can no longer modify? We are going to DL for the first time since paper fast passes and I am trying to learn as much about this as I can. We were always able to maximize the fast passes at WDW but we have not been back since before COVID.