Perhaps my question is wrongfully put to begin with, perhaps there is no such thing as a "sudden change" in history. But I'll ask anyway, and see what comes out of it…
There must have been moments in history where something began/ended "just like that". At some point, WW2 ended in Italy. Someone signed the surrender, and Mussolini was killed. Then what? What happened in practice, did someone walk up to the soldiers that were in the middle of war and said "Guys, you can take down your weapons and go home. It's over, the war is over. Thanks for you participation"?
At some moment in history, the abolition of slavery in the USA was written into law. Then what? Did the marshalls of all southern states walk up to the slave owners and said "Hi, Mr. Man, I'm here to release your slaves." and then said to the slaves "You're all free now. Have a good day"?
Could you please explain how these things are done in practice?
The biggest problem you have to consider is communication, just because an event takes place in place A, doesn't mean people in place B are aware of it. In modern times the technology has been developed to make news spread across the globe in no time but the further back in time you go, the more problematic communication was.
Let's apply this to ending a war for example, just because 2 sides signed a cease-fire doesn't mean the cease-fire took effect immediately. Even in more recent history a period of time before the cease-fire took effect was added to the agreement so all parties involved could communicate it to their troops. But even then the news didn't always reach everyone in time.
For more detailed explanations I suggest you ask more focused questions about specific events.