How fast does this happen? I presume not instantaneously, or I simply fall half a kilometer to the sea's surface and die on impact in the middle of the North American Ocean. Even going slow it's going to be a terrible transition - I imagine only those on ships are going to stand a chance of survival. Unless part of the magic behind this change causes us to all get teleported onto the new land in the process?

Its instant, but everything on the earth, including you, moves with the change, so you don't fall, you just end up at the new elevation.

I think we have to assume that the water behaves the same way and thus has to pour down into the new lowlands, simply to avoid most of use teleporting to the bottom of the sea. Then again, that would be pretty catastrophic. It might be better if the water teleports to its new at rest position and only coastal people survive swimming out of the water, because the entire ocean pouring across the earth is probably going to wipe everything out.