Apple is expected to launch two new iPhone models in 2014, a 4.7-inch iPhone 6 and an even larger 5.5-inch “iPhone Air” phablet. Nothing is official yet, of course, but the months-long flow of rumors, leaks and solid reports has left consumers clamoring for new iPhone models with bigger screens and a sleek new design. Everything we’ve seen so far, from analyst estimates to reports of massive order volume with Apple’s manufacturing partners, has suggested that this year’s iPhone launches will be the biggest to date — and now we have yet another indication that iPhone 6 sales are going to be through the roof.

FROM EARLIER: iPhone 6 release date may have just been revealed

Watching smartphone trade-in patterns is one of many ways we can judge the demand for upcoming smartphones. In the case of the iPhone 6, people who purchased the iPhone 5 on contract two years ago are nearly due for an upgrade. If they begin hanging onto their phones instead of trading them in for a new model, it’s likely a good indication that they’re waiting for Apple’s next-generation iPhone 6 to launch in the coming months.

And it looks like that is exactly what is happening right now.

Used smartphone buying service UpgradeSwap recently shared some data on used iPhone sales through its service over the past few months. While trade-in levels have varied for each model, we can clearly see that iPhone 5 trade-ins as a percentage of all iPhones traded in on UpgradeSwap’s site have plummeted over the past few months.

“After looking at the data, you can see that iPhone 5 users are selling their phones at a lower rate as each month goes by,” the company said in a blog post. “The iPhone 6 is supposed to be released in September and since most iPhone 5 users will be upgrade-eligible come September, we expect most of them to upgrade. Expect lines and chaos as many people will be waiting for iPhone 6 to be available for purchase.”