One of my NASA books talks a bit about things they did. A lot of it was to replace flammable materials with less flammable ones. I think they replaced the insu

One of my NASA books talks a bit about things they did. A lot of it was to replace flammable materials with less flammable ones. I think they replaced the insulation on all the wiring from PVC to fluoropolymer. Changed the design of the connections so there was no exposed bare wire. They redesigned the flight suits which were made partly of rubber. They changed the life support to keep the partial pressure of oxygen constant. So 21% on the ground constantly adjusted to 100% as they went up. For weight reasons the command module designed 5 psi or some such. So that's what they had to do.