I can confirm that this was possible for me yesterday - had a flight from T2 but after completing the security there you are able to use the "flight connections" route to access other terminals. I wanted to head to T3 Centurion Lounge and although it was a bit of hassle (and around 40 mins return) it was possible. You have to clear security again on the way in to T3 and then on the way back to T2 so I'd only advise it if you arrive really early for your flight.