Credit Suisse estimated that 1.4 million AT&T subscribers will switch to Verizon and cause flat sales for the year. Verizon Wireless currently has 93 million customers and the largest wireless voice and 3G data network, according to the company.
Consumer analysts are also watching whether the Verizon iPhone 4 will have similar antenna issues as the AT&T phone last year.
Apple said it made changes to the Verizon iPhone antenna to be compatible with the CDMA network.
Paul Reynolds, electronics editor with Consumer Reports, cautions that Verizon users may want to wait for another version of the iPhone in case Apple releases another phone in June, as it has annually since 2007.
"What Verizon iPhone owners right now will be getting is a six-month, middle aged phone," said Reynolds, who said one year is the average life-span of a phone. "Almost certainly, you're locking yourself into a two-year contract before Apple traditionally rolls out its new iPhone."
Other tech analysts had hoped the new iPhone would have a permanent fix to the antenna and utilize a faster network. Technology companies have touted the growing use of the 4G network at the Las Vegas Consumer Electronics Show.
Reynolds said it is too early to say whether the additional iPhone users will slow down the network as AT&T had experienced.
"Particularly in data, Verizon is in a much stronger position than AT&T has been," said Reynolds. "There are reasons to think as Verizon says, they're ready for this. On the other hand, we know iPhone owners are very or a certain number of them are very heavy data users. Some of that is unpredictable. "
Aaron Katersky contributed to this report.