Are we sure about that? I seem to recall hearing, at the time of the original production, that Marcy Park was the first character in Broadway history to be written as specifically Korean-American. Or the first character in a musical, maybe. Something like that.
That said, I imagine that the authors have provided alternate lines/lyrics for productions that don't cast that way for whatever reason. Even the authors of A CHORUS LINE provided alternate lines for a non-Asian Connie and a non-African-American Richie.