your title and comment are asking different questions. Which one do you want answered?

historically republics have been under threat from victorious generals who were lifted to general acclaim to a kingly position and those can often be peaceful as old rulers either flee or realize the hopelessness of their cause.

As for the United States specifically it is constitutionally impossible for the US to change to a different form of government except perhaps parliamentary democracy (via a really long amendment or more likely a couple of amendments at a national constitutional convention which has always been allowed by never exercised) since there is a clause guaranteeing a republican form of government to every state. That being said America's founding provides an alternative path to change: just ignore the constitution and create another one (constitution went into effect before articles on confederation said they should)

so no protocol and really no possible protocol thus guaranteeing any change by necessity be either quick or a new constitution.

Historically this is fairly common. Czechaslovakia and Spain were good recent examples of a shift to democracy without significant violence. South Africa also managed it with minimal violence.

I think the closest comparison in the US case was the adoption of the current constitution. There is no system for dissolving the existing dispensation. It is about getting a consensus around the new regime. In the 18th century a series of state conventions did that. Today you might need a referendum or the support of all the legislatures.

There is some material from the archives on ratification here http://www.archives.gov/education/lessons/constitution-day/ratification.html

Pauline Maier Ratification (2013) gives a fuller account.

How such a change might come about is too hypothetical/political for this sub reddit but I'd guess a major economic crisis would have to be involved.

Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again. Alternatively, questions of this type can be directed to more appropriate subreddits, such as /r/history or /r/askhistory.