Apple is starting an iPhone trade-in program this month aimed at getting users to upgrade to the iPhone 5 and turn in older models, people with knowledge of the plans said. Apple has teamed up with Brightstar Corp., a mobile-phone distributor, to run the exchange program, said the people, who asked not to be identified because Apple hasn’t publicly announced the plan. Brightstar also handles trade-ins for AT&T and T-Mobile, as well as other carriers and device makers, amid brisk demand for refurbished iPhone 4s and 4Ss in emerging markets. By offering money for older smartphones, Apple and the wireless companies are seeking to entice consumers to upgrade to the latest models, while profiting through resales abroad. “The overall size of this market is increasing rapidly,” said Israel Ganot, chief executive officer of Gazelle Inc., an online mobile device trade-in company.