The US Constitution is so sclerotic it still doesn't have an Equal Rights Amendment; were the Old Country to turn into a de facto[0] dictatorship it would probably be by other means than constitutional amendment. Look at Imperial Rome: they kept the form of the Republican legislative body, yet demonstrated its value by appointing[1] an actual horse to it...
[0] if needed, de jure soon after merely by retroactive application of new law; cf https://en.wikipedia.org/wiki/Night_of_the_Long_Knives#After... and Ultima Ratio Regum.
[1] [Edit: Caligula only threatened to; never apparently did: https://en.wikipedia.org/wiki/Incitatus ]