But the bottom line is that there really is no such thing anymore as a "simple" upgrade to generate a new airplane.
I first became a DER in 1988, and KenV is spot on. The certification efforts required have mushroomed, and they've made life so unpleasant for DER types (I'm now an "Authorized Representative" or AR, but that's just the delegated equivalent of a DER) that I don't think I'd encourage a young engineer to follow the DER/AR career path.
Worse, it has almost nothing to do with safety. Less than 10% of the additional effort is going towards actual improved safety, most if it is bureaucratic BS
