When you need to deploy license key first time, hardware ID on license key should be matched.
Hardware ID might be changing time to time because MAC address is part of hardware ID.
MAC address might be changing on some specific scenario, such as moving the appliance to different ESX, restoring virtual appliance from backup, and so on.
The license file on the appliance will automatically be updated with the new hardware ID on startup, and PAM continue to be functional.