FCOM POLAR NAVIGATION
When reaching an area where the heading reference should be changed (oceanic area, airway or polar track defined in TRUE reference ...); SELECT TRUE by pressing the NORTH REF pushbutton.
However it also says:
if an automatic MAG/TRUE switching occurs when entering the Polar zone, the autopilot might disengage due to the fact that, most probably, the automatic switching will not occur simultaneously for the 3 IRSs. The AP OFF warning is displayed on the ECAM.
I'm not sure what the conditions are for automatic switching as I've always hit the button beforehand.