Sprint Nextel Corp.’s aggressive shutdown of its Nextel network is costing the wireless carrier customers as it grapples with an overall network overhaul. Overland Park-based Sprint (NYSE: S) is deactivating its older 2G network to help cut costs and improve the efficiency of its network, which is undergoing an extensive upgrade to 4G LTE technology. The project is called Network Vision. On Thursday, Sprint reported an increase in overall subscribers in 2012 — 55.6 million subscribers, up…