EVERY once in a while an e-mail lands in the in-boxes of journalists who cover the Metropolitan Opera, with the subject line announcing a “cast change advisory.” The e-mails are simple and straightforward, and they tend to make people very unhappy.


“Frank van Aken will make his Met debut as Siegmund in ‘Die Walküre’ at tomorrow’s matinee performance,” began a typical one from April. And then the agonizing kicker: “replacing Jonas Kaufmann, who is ill.”


Sometimes the e-mails do not even give that cursory excuse. Like this one from May, part of a litany of cancellations for the coming Met season, which opens on Monday with Donizetti’s “Elisir d’Amore”: “Sondra Radvanovsky will replace Karita Mattila as Amelia in Verdi’s ‘Un Ballo in Maschera’ on Nov. 8, 12, 15, 19, 24, 27, 30, Dec. 4 and 8 matinee.”


In the space of a few words, the leading role in a major new production had been reassigned. But why?


The e-mail did not say, but the news was significant enough to prompt a follow-up post on The New York Times’s ArtsBeat blog. There a Met spokeswoman, Lee Abrahamian, said, “Karita was engaged for ‘Ballo’ five years ago, and she recently decided the role was not right for her.”


In the euphemism-heavy world of opera cancellations, this was, finally, something akin to plain speaking. It also gave the public a rare glimpse into the byzantine world of opera casting. Many audience members are unaware that operas are planned, and largely cast, five years ahead. This longstanding policy, which has hardened into a simple fact of life, creates stability for artists and companies, a boon in troubled economic times.


“The main advantage is being able to lock in the top talent in a world in which it’s necessary to act five years in advance,” the Met’s general manager, Peter Gelb, said, showing a spreadsheet with the company’s plans for dozens of singers through the 2017-18 season. “Ideally we wouldn’t have to act five years in advance. We’d act 1 year in advance, or 2 years, or 18 months. But the world in which I arrived when I came to the Met was not going to change.”


The system provides security, but it also tends to make opera companies slow-footed. And it leaves enormous room for making incorrect assumptions about what a voice, that most fragile and changeable of instruments, will be able to do five years down the road, even though singers and companies tend to be flexible when it comes to breaking contracts. Hence someone like Ms. Mattila, deciding that a role is no longer right for her and causing a company to scramble for a replacement. Or the even worse cases in which singers make that same determination but then proceed to sing the roles anyway.


“I had to cancel my first Adalgisa in Fort Worth years and years ago,” the mezzo-soprano Joyce DiDonato said of a major role in Bellini’s “Norma.” “As it got closer, I just didn’t have the time to prepare that I thought I would. It was too much, too big. So I had to cancel with not quite six months’ notice, and that’s not like me.”


Things used to be different. Rudolf Bing, the imperious head of the Met from 1950 to 1972, would cast the next season while on tour with the company in the spring.


“It had a lot to do with the increased availability of air transportation,” Alexander Neef, the general director of the Canadian Opera Company in Toronto, said of the shift in planning. “All of a sudden people realized that they could be many places within a few months. Companies realized that artists who had seemed to be inaccessible were now accessible, so it became a race.”


Air travel contributed to a stretching of the planning cycle, but even by the end of the 1960s it was just a couple of years long. Matthew Epstein, a veteran artist manager and opera house administrator, said that a single influential figure was responsible for the jump: Joan Ingpen, the administrator in charge of the conductor Georg Solti’s artistic team at the Royal Opera House at Covent Garden in the 1960s, who died in 2007.