The change over was still happening at some parks as late as Nov 15th this past year. It was pretty cool to see at Epcot, one day it was all beautiful flowers and fall colours, the next it was pointsettia and christmas decor everywhere.
I would recommend staying through the 9th because the 7th is a Monday and parties aren't typically held on Mondays and I think before the 7th is too early for the first MVMCP. (Although I would certainly expect there to be a Halloween party on Halloween itself, which is on a Monday this year.)
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