Last-minute cast changes at opera houses are always full of drama, but the one that took place in Saturday afternoon’s performance of Puccini’s “La Bohème” at the Metropolitan Opera promises to go down in the annals of day-of-the-performance substitutions.

The rising soprano Kristine Opolais, who had just sung the title role of Puccini’s “Madama Butterfly” for the first time at the Met on Friday night, was awakened at 7:30 a.m. Saturday with a phone call from Peter Gelb, the Met’s general manager, who made a bold request: Could she possibly sing the role of Mimi that afternoon in the matinee of “La Bohème,” to replace an ailing Anita Hartig?

“It was crazy,” said Ms. Opolais, who added that she had not fallen asleep until around until 5 a.m. after all the excitement of her “Butterfly” performance. “I said, ‘No, no, no — it’s impossible. I would love to do that, but it’s impossible.’ But then something happened, and after 5 minutes I said, ‘Why not?’ ”