Sometimes hat fashions extended from one era into the next. It was common for the decorative pieces to be modified over time to give an old hat a new look.
There were many hat styles that appear during each period. You will see several examples of these varying styles and may notice the transition between styles and periods can be somewhat blurred...as an example you may see a lady wearing a Civil War period dress with a hat that mirrors the same style of a hat worn by another lady in the early Bustle period. It appears hats were not necessarily discarded as the dress styles changed, they were apparently redecorated for use with the new fashions.