You might think that, after performing some 92 roles onstage (123 when you count recorded roles) over a career that spans some 48 years, the world's most versatile operatic singer might now allow himself to draw breath and contemplate a more rallentando role.

But no. Already officially a pensioner - he celebrated his 66th birthday on January 21 - Placido Domingo has announced to the world that he is to up the tempo dramatically, and con brio, to boot.

In 2009, when he is 68, he will take the dramatic leap from being a tenor to being a baritone when he takes on the role of the Doge of Genoa in Verdi's Simon Boccanegra at the Berlin Staatsoper.

And this is not just any old role: this is one of the most demanding of the baritone repertoire. Natch.

This decision has (predictably, given that we're talking the über-bitchy world of opera) prompted cries of "bravo" from some quarters and "boo-hiss" from others. On the one hand, Domingo is, indeed, being rather noble: in opera, the tenor always gets star billing alongside the soprano because he takes the heroic or romantic lead (think La Boheme, Tosca, Rigoletto, Butterfly), whereas the baritone role is traditionally that of the baddie, full of deep-voiced threat and menace. So Domingo knows he is, to a certain extent, stepping out of the populist limelight.

On the other hand, however, some people are wondering why the Madrid-born superstar does not give up now and retreat with dignity while he can. The voice is not as it once was when, in 1960, he first appeared as a young Alfredo in Verdi's La Traviata, and went on to master such lyrical roles as Otello, Don Carlo and Sigmund in Wagner's Die Walkure in his internationally admired coppertoned timbre.

It is only 10 years since he appeared at Covent Garden singing the tenor role Boccanegra, but since then, his voice has been gradually becoming darker. That, say some, is putting it politely.

But Domingo - who in 1958 provided backing vocals for a Spanish rock'n'roll band called Los Black Jeans - has made a virtue of his chameleon-like changeability and clearly likes to show off his stamina.

No other tenor has demonstrated such ambition with the operatic repertoire (unlike, say, his Italian counterpart Luciano Pavarotti, who steadfastly stuck to the Italian bel canto roles with which he began his career).

In 1987, he even sang a duet with the late American country/folk singer John Denver, and with Denver and Julie Andrews sang in the 1987 TV recording of the Sound of Christmas. And, of course, with Jose Carreras and Pavarotti he was one of the Three Tenors who first sang at the opening of the 1990 World Cup in Rome.

In the end, however, his move to baritone is not really a surprise, because it was as a baritone that he started out in 1959, in Emilio Arrieta's Marina at the Teatro Degollado in Guadalajara, Mexico.

Domingo's "final career move" will take place exactly 50 years after that fateful event, so perhaps it is his vocal band of critics who should stand back and allow him to come full circle at his own tempo.