The first change was the U.S. Mint removed the initials if Victor D. Brenner, the creator of the penny. The reason was Americans complained the initials were huge. The reaction of the americans was they felt felt heard and understood. Another result of this change was 1909 penny became rare.
Another adjustment was U.S. Mint switched to components of the penny from tin/zinc to steel. Tin and zinc was wanted to make war material. One outcome was the penny to dull at a faster rate than tin. However, americans felt proud to help the war effort.
The final revision that the U.S. Mint decided