UBS analysts Steven Milunovich and Peter Christiansen believe Apple will sell 2 million more iPhones than previously estimated this fiscal year, and they have raised their December sales estimate to 56 million units in total due to an unusual wave coming in terms of customer renewals.

Eighty percent or more of iPhone users have old phones or are due for an upgrade, they say.

These two charts tell the tale.

Basically, more than 50% of iPhone users own the tiny iPhone 4 or iPhone 4S version of the phone. That model is years out of date and the big-screen iPhone 6 will be a huge temptation for those customers to renew. Once you add in iPhone 5 users, it turns out that about 80% of iPhone users have a phone that is at least two years old.

That just looks like a giant wave of people who will want a new iPhone when the iPhone 6 is unveiled in fall:

iphone UBSUBS

In terms of wireless contracts, 97% of iPhone users will be eligible for some kind of ugrade, too:

iphone UBSUBS

The UBS analysts also note that China will fuel iPhone growth, when previously it did not:

Accelerating penetration with China Mobile and strong developed country interest in the iPhone and new products may help to improve the company's fixed cost leverage.

The pair believe Apple stock will hit $700 on the sales. It's at ~$635 right now.