I've noticed that when they come in empty from T4 they show "8Axx" but as soon as the doors close at Heathrow Central, it changes to "9Txx".
I'm not sure if it's the driver that does the change.
The driver can set it up to do that when setting up the cab for an ECS trip followed by a passenger service.
Alternatively it can be changed by the driver manually when necessary.
However there was a glitch in new software which meant the trains wouldn’t recognise where they were and defaulted to locking out doors automatically when using the preemptive headcode. As trains reversed at T4 this meant only the middle 3 carriage’s doors would be unlocked.
This may have been fixed if you have witnessed the headcode change straight after doors being released. I haven’t been brave enough to try it out again.