They did the same thing that Jaguar did, when they went from the late-70s underpinnings of the X308 XJ to the all-new X350 XJ in 2003. The style cues...the first things that meet the eye; the curve of the rear fender and the shape of the headlights-those things were kept-but the proportions of the car are different. The new one was taller and narrower, with a proportionally smaller greenhouse. It takes a while to see it; but it's a dramatic difference when you do.
The Charger/Challenger were the same way. Th '70s originals were built on the old Dodge Coronet platform-low and wide. The new (current) ones, built on the Chrysler LX platform shared much of their hidden sheetmetal with the Mercedes W210/W211 E-class; so instead of low and wide, they're tall and narrow, more like a German luxury sedan. Without, you know, the good visibility.
It may still have been tricky to get regulatory approval, though. I can’t share my sources or specifics, but I do know stories of strings being pulled and crash tests being fudged in order to get a muscle car’s new model cleared to sell.
Safety standards are getting strict enough that they do influence the physical shape of the car, because the physical shape of the car influences your options for how to dissipate energy in a collision.
Certainly, if you look at them side by side, the new Challenger is noticeably "thicker", for whatever reason, no doubt mostly regulatory since we know how much manufacturers enjoy adding weight, especially to performance cars. But at a casual glance, it still looks more like a Challenger than not.
They'd probably have real problems with a Barracuda.
