Counter-Errorist
THINK TANK
Radar on upgraded ones - APG-83 ?When an F16 is upgraded, the only thing that remains old, is the engine, per se. Although after a complete overhaul, it is nearly a new engine.
The physical airframe gets an SLEP i.e. Service Life Extension Program, which physically strengthens the airframe life up to 12,000 flt hrs.
Then comes in the cockpit, radar, and other sensors / avionics.