I've been wondering since reading the book... Why the switch from Nick Chopper in the book as the Tinman to Boq in the musical?

This was me too, again, not sure how I got logged out. lol.

Just as Fiyero is created into the Scarecrow, Boq replaced Niccolo as the Tin Man to give connection to the events. Previously seen in other books that deal with "what happened before Dorothy", some people feel the overwhelming urge to connect the three males to their stories. By making Boq into the Tin Man, to my knowledge, this will give the man of Tin a reason to hate the Witch.

Nick Chopper had no connections to the Witch of the West. His only connection to the Witch of the East was that she was responsible for his tin limbs. Instead of trying to throw Nick and Nimmie Amee into the story, it would save time and sense to simply make Boq into the Tin Man.

It's the same situation with the Scarecrow. They wanted to add sentimental value to the story.

That makes sense. Thanks :-D