We had a wonderful house that was on a wagon that we used during the tornado scene and in the final scene.  I got the three leads off ASAP so they could run back and change. I had another cast member waiting with a sponge/washcloth and sent each one to a sink backstage where they had help washing off make up and quick changing into their farmhand clothes.  The only people left onstage for that last scene were Glinda and Dorothy.   When Dorothy clicks her heels, the music for that transition is at least 3 minutes...   My torndado kids (dressed in all black from head to toe) came in and turned the house around and around on stage while moving it from SL to SR.  The back drop was changing, too.  Dorothy slips out of the light (for her quick change into black and white) and then I had three ribbon dancers (that were used in the tornado scene from act 1) dance for about 90 seconds with the transition music.  They were really cool... had long black dowels with satin ribbon draping off the ends and they twisted and danced until they slithered back into the dark and the lights came up on the house.  I had Uncle Henry and Prof. Marvel take their time with their scene, then Aunt Em... and the three leads straggled in one at a time after that. They were amazing at their quick changes... and we are talking teenagers (youth theatre).  Sometimes the girl who played tinman had silver on her neck... or the lion had some brown on his nose... but it added to the charm. The audience WANTS to see the same people that were those three come back on stage as the farmhands... that is part of the story that we all know!