WHAT'S this then? While the Baron of Lan Kwai Fong, Allan Zeman, has been away the MTR has sneakingly changed the name of Hong Kong's night quarter. For the MTR sign at Central now directs travellers to ''Lan Kwan Fong'', which the chairman of the Lan Kwai Fong Association Richard Feldman figures is the Mandarin spelling. Says he: ''It seems even we are getting ready for 1997.'' All the same, with that date still some time away the MTR sign we understand will revert back to the spelling as known by us all - although Keeping Posted, shame on us, spelt it Lang Kwai Fong the other day. Thank God Zeman is still away.