I would say the 13th, 14th and 15th amendment, the Civil Rights Act of 1875, the various enforcement acts in the early 1870s, etc. did overhaul the system of government, by changing what it meant to be a citizen, defining people's rights more explicitly, and outlawing slavery. Given that black people made up about 20% of the population, suddenly (theoretically) allowing them to vote and giving them the rights that non-slaves had was a pretty major change.