As I recall Lockheed's 'simple' upgrade of the C130 into the 'J' was over two years late and well over budget.
Part of that was due to the certification issues. Since the J was a company funded effort certification was civil. The upgrades from H to J were sufficient that the FAA would not "grandfather" the many faults/problems of the old Hercules to the new one. So a lot of the delay and expense was to solve problems with the old airplane that were no longer considered acceptable. Stall characteristics were bad and required addition of (among other things) a stick pusher. Icing characteristics were bad and required many changes, including the addition of an anti-ice system on a section of the vertical fin/dorsal. Fire suppression in the engine nacelle was inadequate and required changes. Going to a two man cockpit required the addition of lots of automation, but the crew workload was excessive during emergencies and the avionics required lots of rework. In short, the "simple" upgrade was not at all simple and the original budget and schedule were woefully over optimistic. And even with all this hindsight, there were lots of unexpected issues when applying the C-130J upgrades to the C-27A to make it a C-27J. The 737MAX is a "simple" upgrade of the 737NG, but it still requires lots and lots of engineering and testing. The same with the A320neo. Indeed the problems there have been sufficient that Qatar has decided to walk away from their A320neo order. Pretty drastic action for a launch customer. In today's environment, there is no such thing as a "simple" upgrade for a new airplane.
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