What's the condition for detecting a lifted building? Can I force the building to go into land mode or I need to remove & then create?

To detect a lifted building, make a Location on the size of the building, go to its properties, and uncheck all "Ground" boxes (or at least I think what they're named). Then, place it on the building you want to monitor, and run a trigger to detect when the building owned by the corresponding player is present in the location. Since you unchecked all the Ground markers on the location, it should only be able to detect the building if it's in the air. If the building lifts, I think you have no other options but to remove and then recreate the building.

Make a location and uncheck all the ground flags. Keep only the "low air," "medium air," and "high air" flags checked. You will have to remove the building and create a new one if it's lifted; you can't force a landing.

In the event the building could be anywhere, you can just make one location the size of the map with all the Ground boxes unchecked.