-- Effie's ending 'And I am Telling You' segueing to the new Dreamgirls appearing on stage has got to be on the top of any list. The audience was deprived from raining the roof and that made it more exciting.
-- Agree re the original production of Follies transitioning to Dreamland. Incredible, theatrically historical moment.
-- Agree re the citing of the mansion in A Weekend in the Country, for my money one of the greatest numbers Sondheim ever wrote and Prince ever staged. An entire story in a single number, staring in multiple homeland ending in the country.
-- I don't know if this is the qualifies as a scene change but, for my money, one of the most exciting moments in musical theatre is when the train pulls into and out of the station during Put on Your Sunday Clothes.
-- There was an incredibly magical moment in Nicholas Nickleby, when Nicholas takes a ride on a coach. The construction of the coach was one of the most magical moments I have experiences on the stage.
-- The end of Nine. Suddenly the white of the stage is emphasized, but is also complemented with an aquamarine glow across the stage. As the music is crescendoing, doves are released into the auditorium. As beautiful a stage image as I ever expect to see.
-- One of the most boring evenings I ever spent in the theatre. The show was Agamemnon at the Vivian Beaumont, directed by Andrte Serban. The entire audience is on multiple moving seating areas (think stadium sections, but movable. Several scene changes were made not by moving scenery, but by moving the audience. A moment of incredible creativity in an other wise miserable evening. Perhaps it would have been better if the dialogue was in English OR there were supertitles somewhere. As it is, I seem to think that it was not even in Greek, but rather some made up language. I could be wrong there.
-- The first time you see the mansion in Sunset Blvd. Naysayers say the set was too overwhelming and probably contributing to the show's financial failure; however, that was a very exciting moment.
-- The house comes tumbling down in An Inspector Calls. A very impressive design feat and perfect in the context of the show. There was a similar scene transition in Indiscretions, a decade or so later I imagine, but it would have been more effective if one did not have the memory of An Inspector Calls.
Lots of others, but I just can't think of any that are as impactful as the ones above.