Several top investors in Twenty-First Century Fox Inc are pressing for the right to swap their voting shares for ordinary shares, which are trading at an unusual premium, even though the move could hand even more control of the company to Rupert Murdoch, according to people familiar with the matter. Fox’s dual-class share structure already gives the 83-year-old media mogul control over 39.7 percent of voting rights, even though he and his family hold only a 12 percent equity stake.