iPhone 6 and iPhone 5

Sprint is already preparing for next month's launch of the iPhone 6s with the introduction of a new plan called iPhone Forever. Simply put, Sprint customers can upgrade their current iPhone to the new model at any time, and still retain their current monthly rate.

Sprint says:

Starting today, new and upgrade eligible Sprint customers can get iPhone for just $22 per month with iPhone Forever. Anytime customers don't have the latest iPhone, they are eligible to upgrade. They bring their iPhone, upgrade on the spot and away they go. It's that simple. iPhone Forever is available on any eligible Sprint rate plan and upgrade eligibility is always included in your price.

The $22 a month rate is for a 16GB iPhone 6, so the rates will be higher if you have one with more storage or if you have the larger iPhone 6 Plus. Sprint is also running a special promotion that will cut down the $22 a month rate for the 16GB iPhone 6 to just $15 a month for current or new Sprint customers.

New or existing customers who are upgrade eligible can bring in any smartphone and get a 16GB iPhone 6 model and their monthly rate will be reduced to just $15 until their next upgrade. As a special promotion, customers who get iPhone 6 and upgrade before Dec. 31 continue with the discount price of $15 until their next upgrade.

Source: Sprint