What gets somewhat confusing is what is grandfathered and what level of change triggers the requirement to bring things up to current code. The world didn't jump right from notching sticks to using computers to do accounting, for example...over the years, we've learned from our mistakes and the codes have been adjusted to account for newer materials and previous inadequacies. While the 'older' ways may have worked, they may not be all that safe. Think about knob and tube wiring...technology didn't exist to use stuff like romex...nobody in their right mind would install the stuff today, but it was done, and is still in use in places, but any changes dictate updating things.