I also remember reading that they kinda knew Phantom would run a long time therefore they built alot of the set to last. (steel stage etc.)
I don;t think they changed the staircase as that would be a huge changeout, and its purpose built for that theatre so they cant just swap it with the old one from tour. They might change bits and pieces as needed and repaint. You can also see alot of areas on the stage floor that gets repainted or replaced. Where the chandelier sits is very scratched up and the floor of the travelator could also use a coat of paint when last I saw it (because of the little seen rat catcher thing)
Funny about the Wicked bubble, if that was true. Last time at Phantom it looked like James Barbour could barely fit into that cross in the mausoleum scene.
I;ve also said alot that phantom London needs alot of work, I know they replaced drops and such but compared to Broadway, the london version is starting to look old and shabby.
http://www.flickr.com/photos/27199361@N08/ Phantom at the Royal Empire Theatre