The attractions that would appear to need downtime are:
Dumbo (to move the spinner)
Carousel (If it's still moving)
Mad Tea Party (If the roof is changing)
Barnstromer (if it's being rethemed)
Considering we have a 3 year window for construction, I would imagine that the downtime for these would be staggered, and none of it would be substantial. As I said elsewhere, I wouldn't be surprised if they open up Dumbo in it's new location with one spinner, the same day they close Dumbo in the old Location. To get an idea on the spinner time frame - how long was the Astro Orbiter down at Disneyland? They completely dismantled it recently and put it back together.