[WilliamDraco]

within 8 hours (1 in-game day)

One in-game day is 6 hours

Will furniture that's been placed be fine for the migration, or should they be taken out too?

Furniture that has been placed is expected to be A-OKAY for the migration, unless something goes wrong of course  
Unless of course it is furniture with storage, in which case clear that out (the furniture itself will likely survive, but the contents won't)

Would you recommend unloading these for now and removing them from buildings?

Hope is for it all to go smoothly, it's just opening probability for errors/mistakes/something to be missed.
If you want to be careful, you could remove them beforehand, but it's not strictly necessary. Balanor's been doing the testing though so he will have a better feeling on confidence.