Sorry if I am repeating a topic but just curious did you ever see a play or musical in out of town try outs or previews that changed drastically for the worse on opening night and if so how?
I can't think of any shows that fit this description for me, but I can think of many individual performances that fit the category.
"You travel alone because other people are only there to remind you how much that hook hurts that we all bit down on. Wait for that one day we can bite free and get back out there in space where we belong, sail back over water, over skies, into space, the hook finally out of our mouths and we wander back out there in space spawning to other planets never to return hurrah to earth and we'll look back and can't even see these lives here anymore. Only the taste of blood to remind us we ever existed. The earth is small. We're gone. We're dead. We're safe."
-John Guare, Landscape of the Body
Jekyll & Hyde had 3 out of town productions prior to Broadway. The first two were great, despite major changes in score and staging with the second being my favorite (the original was designed for a theatre with a deep thrust while the second was designed for oriscenium, preparing for Broadway) but the third started to deteriorate a bit in staging, book and score. All three were far superior to Broadway which was drastically different and laughably bad by comparison. The only improvement Broadway made was to the orchestrations.
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian
The Seattle pre-Broadway tryout of CATCH ME IF YOU CAN was far superior than the Broadway production. The staging concept in Seattle was fantastic with the huge vintage TV screen backdrop and the orchestra on a swoop. It felt like you were watching an early 60s technicolor NBC variety show. They slicked it up and polished it up for Broadway which took away the show's uniqueness. On Broadway it looked like a horrible late 70s awards show with all that white.
I thought some of the changes they made to Finding Neverland during previews were for the worse, but again, I don't think that really fits the OP's question of a show that was "great" before opening and "bad" after.