When I changed my HU couple of weeks ago (disconnected the battery), when i finished everything connected the battery and started my car without being required to enter the code but I also could not arm it, as in the keypad was not lit. Note though that my keypad was in manual arming mode. Battery was disconnected for about 5hrs (very slow Saturday).
Saturday started and turned off the car plenty of times, Sunday tried to start it and I was required to key in the code. I tried to key in my code and it wasnt accepting it. Manage to trick the Brant into not asking me for a PIN, but then few days later it happened again. Called up Subaru Melville, and after proving that I'm the owner of the car and providing the VIN, they gave me the master code.
Had to wait till Brant would require me to enter the code again, entered the master code and after that my old code worked.
Sorry about a novel, but I saw that heaps of people had issues with Brants so I thought to post my experience. I would suggest trying to get a master code for your car if possible before disconnecting the battery.

Adi