The wireless giant doesn't see iPhone upgrades being a problem.Wireless giant AT&T (T 0.98%) has an enormous base of iPhone users, many of whom are using older models. During a recent conference, the company declined to disclose exactly how many iPhone users are on its network. But with the iPhone having a market share above 50% in the United States, and with AT&T having nearly 90 million postpaid phone subscribers, we're likely talking about many tens of millions.Each year, Apple (AAPL -0.36%) releases a n ...