From what i can understand, the change over pretty much happens over night, the night of the last Halloween party (which is actually on 10/31 this year). There is a show on Freeform called Decorating Disney-they did one a few years back about Christmas decorating and another one last week about Halloween decorating. It shows just how quickly they get a majority of the decorations up and it is honestly super interesting. But they mentioned that for MK at least, they wait until the last of the guests have left the park the night of the last Halloween party, and then a crew comes in and puts up most of the decorations before park opening. As for the other parks and the resorts, i have heard they are decorated by mid November.