IT'S all change at RTHK yet again as it embarks on a search for a talking head to replace Cindy Wieringa, Radio Three's bubbly lunchtime show host. Having taken over the slot from Anna Chilvers a few months ago, Aussie Wieringa has heeded the academic call (although many who work at Broadcast Drive end their time there claiming they have learned a good lesson!) and is departing to take up a post-graduate course in broadcasting at Cardiff University in Wales. It must be down to something in the coffee served at the Radio Three studio, because a previous lunchtime host, Valerie Whitehead, also left some time ago to take up the same course at the British university. The task of finding a replacement falls on the shoulders of Jagjit Dillon, the head of RTHK's English programming. He told Keeping Posted he was looking for someone ''with a lovely voice, radio experience, a pretty thorough knowledge of Hong Kong and, of course, bags of personality''. So don't all rush. While on the subject of RTHK's search, we might also mention that the Watershed Theatre Company is looking for two male actors, aged between 20 and 30 and 45 and 60, for its production of A Life In the Theatre in November. Auditions will take place for the two-man play on September 17 and 20, with rehearsals starting shortly afterwards. For more details contact Bob Machin, the theatre company's producer, on 813-0694 (phone) or 813-6463 (fax).