Definitely go to the airport. But to make it easier change your reservation to the new desired routing before going to the airport. And bring your PNR with you.
I have had QF ticket desk at LAX do this for me in 30-45 mins (although they clearly didn't know what they were doing). Alas you won't be in LAX. NOt sure about ticket offices at other airports - apparently you should avoid BA as they charge an extra fee over the $75 as per the fare rules.