This really depends on where you're located and what type of 100-year-old house you have.

Do not listen to anyone who hasn't worked on the same type of old houses in your location, or at least in a similar climate. You can absolutely destroy a 100-year-old house by retrofitting it with modern materials and/or using modern building methods. You can also cause serious damage by using methods and materials that work really well in a different climate.

Tons of houses in New Orleans were destroyed within a decade after hurricane Katrina, when contractors came from all over the US to cash in. They used methods that are perfect for modern houses in Phoenix or Dallas or old houses in Philadelphia or Buffallo, but they resulted in all kinds of rot and termite problems here.