BIGBANG's Daesung played Rum Tum Tugger in the Korean version back in 2008.
The Other Marty: Happened in both the original London and Broadway productions:
In London, Judi Dench, the original Grizabella, suffered a hamstring injury, then compounded it by falling off the stage. At that point, Elaine Paige stepped in.
In New York, Don Correia was cast as Skimbleshanks. He left almost immediately, so Willie Rosario, who was understudying the role, took over. Rosario was badly injured during rehearsals, so his understudy, Reed Jones, replaced him and opened the show. Things ended well for Rosario: after recovering, he was given the role in the first national tour.