You cut a giant door into the backside of the building. In theory it would be a lot easier for WDW to completely gut their SM cause the building is larger and it sits on a backstage road with a gate already installed. Would be very easy to get the old track out and new track in.
No need. WDWs mountain was completed as a shell and the ride added afterwards. The large doors at the rear would allow sections to be taken in and out. The TTA and guest areas add a bit more planning, but this was all taken into consideration until the full refurb was scrapped.