A THIEF posed as a customer and exchanged a diamond valued at $773,200 for a fake worth $50 at a jewellery exhibition where organisers had been alerted to security lapses. The swap occurred just before noon on Wednesday at the Hong Kong Jewellery and Watch Fair being staged at the Convention Centre this week. Earlier this week, a security expert criticised poor security at the exhibition and described it as an ''easy target'' for thieves. Loose security was also blamed for $1 million worth of jewellery being stolen from the Ninth International Jewellery Show at the same venue earlier this month. In the latest incident, police said a Japanese man in his mid-30s approached one of the stands and asked to look at four diamonds. After the ''customer'' had left, the exhibitor weighed the diamonds and discovered a 5.74 carat diamond had been replaced with a synthetic stone.