Over 2,000 F-16s are undergoing a structural modification program that will  enable the aircraft to fly their planned 8,000 hours, rather than only about 6,000. Each kit contains new parts and tools and costs about $500,000, and takes several months to apply. The fix consists of reinforcing or replacing many internal structural elements of the aircraft. There will be over 40 variations on the basic kit, to accommodate the many different models of the F-16. It will take about nine years to apply the kits to all aircraft that need them. 

Fixes like this are common with combat aircraft. When the initial design and testing of a military airplane is done, a lot of assumptions have to be made about exactly how the aircraft will be used. Tactics, and weapons carried can change over the 20-30 year life of an aircraft. This will change the kinds of stress put on the aircraft structure. This results in some parts of the aircraft wearing out earlier than expected. This can shorten the useful life of the aircraft, reducing  flight time by hundreds or thousands of hours. Modification programs like this take care of those problems.