If it is not possible to get a replacement, show up at the airport early and explain it. They will mark your ticket as needing extra security screening. When you go to TSA, they will usually do some extra screening including a pat down then let you go. This usually works, but it is better to have an ID if possible. Also, if possible, check that passengers bags to someone else. If no baggage is involved, it usually makes things easier. Remember though, TSA is very unpredictable these days.