Changing issue needs to be addressed
I had a similar experience. We did not change until through all the security check points due to unsat conditions in the bathrooms outside screening. It sucked being in uniform and not going through the crew line in uniform. We had lots of odd looks from security and several "conversations" trying to explain what we were doing. We did change prior to flying which is what the instructions required. I am definitely not looking forward to doing that again.
I've heard others discuss changing on the FedEx airplane prior to exiting.