I don’t know how long it took them to develop, but we know that in other case change scenarios they did it during a normal 12 month cycle. I doubt it takes them 3 weeks or something. When you make things like this in volume, you do a lot of modeling on crack formation, minimizing waste, manufacturability, etc. and you need to do that for each material (aluminum, steel, titanium).
In this case there is the additional wrinkle of completely changing how the crystal interfaces with the case - that’s a big source of potential engineering risk. They also increased dust resistance, presumably via that work.