A few years ago when 4G was just a buzzword and most people had just upgraded to 3G phones, Sprint was heavily investing into a brand new infrastructure that was supposed to take us into the next generation of mobile technology. WiMAX was its name, and it was the direct competitor to LTE technologies that so many other companies had jumped on board for. Much like the Bluray vs HD-DVD war, however, the quickest route to a perceived next generation isn't always the best, and now it looks like Sprint is working to remove over 6,000 sites from its network of leased towers and planning on decommissioning all WiMAX infrastructure by November 6th, 2015.
While this is over a year away it likely can't come soon enough for Sprint, as this infrastructure is expensive to maintain alongside it's existing 2/3g and LTE networks. Right now Sprint is working with its Mobile Virtual Network Operators, or MVNOs, to alert their customers of the upcoming change and that they can do, namely get newer phones. Considering it's been a few years now since Sprint has been selling phones with WiMAX as the primary means of 4G connectivity anyway, and Sprint is assuring customers that everyone who's on a WiMAX phone right now is already eligible for an upgrade, this shouldn't have too much of an impact on many people. Even so if you're still on a WiMAX phone your connectivity won't completely go away since you'll have the existing 2/3G network to fall back on.
Back in Spring 2013 Sprint issued a memo saying that it reserves the right to migrate its customers from WiMAX to LTE as long as they issue advanced warning. Giving more than a year to upgrade your phone seems like more than enough warning enough, so consider it something on your to do list if you haven't moved on yet.