Well, wish I’d looked this up earlier… I have a static IP, so no problems. Went to a LAN at a mate’s place a year ago, so set up my authenticator… and in the meantime got a new phone.
So, with an hour to go until the LAN, I realise there is no way I’m going to be able to access my GW2 account at his place.
As noted above, the remove authenticator button is greyed out, and support tickets are running 1-3 days.
They really need a way to remove old authenticators if you can fully authenticate your account, especially if you’re connecting from the one and only trusted IP address in the account.