Alongside a winning “L’Elisir d’Amore,” our critic returned to four works at the Met in the middle of their runs to hear new rotations of artists.As the first act of Verdi’s “La Traviata” ends, Violetta, a high-end prostitute, is suffering the symptoms of tuberculosis. A well-to-do young man’s declaration of love shakes her seen-it-all cynicism; should she put an end to her life of pleasure and accept him?Screw it, she decides: As champagne fizzes of coloratura rise and rise, she declares that she is “forever free” and brings the curtain down in defiance.At the Metropolitan Opera in November, the soprano Nadine Sierra sang that moment with luxuriant ease and confidence, a woman certain that she still had all the time in the world. On Sunday at the Met, though, Ermonela Jaho — her tone far less plush than Sierra’s and the aria less easy for her — made it a kind of mad scene. Violetta’s fragility, her sleep-when-I’m-dead mania, were scarily center stage.Same words, same notes, an entirely different effect: This is one of the best parts of my job. In addition to attending first nights at the Met with other critics — as on Tuesday, when a delightful revival of “L’Elisir d’Amore” opened with the power to make you giggle one minute and choke up the next — I spent the last few weeks returning to four classic titles in the middle of their runs to see them with new rotations of singers.Especially in the standard repertory, the Met often cycles through multiple casts in a single season — and then does it again, year after year, Violetta after Violetta after Violetta. Ticket sales in this not-quite-post-pandemic period have blinked red lights at this practice. Houses are full for new productions, even of contemporary works. But revivals, less obviously newsworthy and less widely promoted, are no longer certain draws.The tenor Javier Camarena, center, emanates sincerity and modesty in “L’Elisir d’Amore.”Marty Sohl/Metropolitan OperaThere is not the audience there once was to hear “La Traviata” twice over a couple of months. There is not even the audience there once was to hear it twice over a couple of years.Which is something to mourn. Being a lover of the performing arts is about the thirst for the new play and concerto. But it’s also about relishing the Hamlet of cool distance next to the one of slovenly aggression; about how Beethoven’s Fifth Symphony can be noble or ferocious in the hands of different orchestras and conductors; about how each new soprano increases our sense of what is possible in a work, and suggests how capacious we are, too.Not that every contrast is quite as extreme as in the Met’s “Traviata” this season. In November I had been impressed by Sierra, who in her mid-30s is coming into her own vocally. But, as in her sumptuously sung “Lucia di Lammermoor” last year, she has not yet solved one of opera’s fundamental challenges: making rich, ample tone convey desperation, illness and frailty.Desperation, illness and frailty happen to be Ermonela Jaho’s stock in trade. We often hear about opera singers being larger than life onstage; Jaho manages to be smaller, to give the sense of death incarnate, a walking, singing corpse.About 15 years Sierra’s senior, she has a slender, meticulous sound that she doesn’t push to be bigger than it is. Her “Ah, fors’è lui” and “Dite alla giovine” were murmured reveries, ghosts of tone; you got the sensation of thousands of people in the audience leaning in to overhear private musings. I can’t remember experiencing such prolonged passages of extremely soft yet palpable singing in the Met’s huge theater, which artists often think they need to scream to fill.Jaho can be shamelessly old-school; this was probably the most coughing I’d ever heard from a Violetta, and her “Addio, del passato” in the final act milked every wide-eyed tremble and gasp for air. She didn’t summon the fullness of voice that an ideal Violetta requires, at least at certain moments. But Jaho unsettlingly lives this unsettling opera, providing a sensitive, unique vision of a classic.The tenor Ismael Jordi, making his Met debut as Alfredo this season, was on Sunday a gawky more than dashing presence, who spread mellow legato lines like schmears of cream cheese. The baritone Amartuvshin Enkhbat is also making a company debut — this one rather more impressive — as Alfredo’s father, the elder Germont, with his burnished-mahogany “Di Provenza il mar” providing the most deeply satisfying singing of anyone onstage.In another Verdi work, “Rigoletto,” the shift of personnel marked a less dramatic change but resulted in a keen performance. On Dec. 17, the soprano Lisette Oropesa sang Gilda with a tone a few shades brighter and more finely vibrating than the softer-grain Rosa Feola had earlier in the fall.Lisette Oropesa, left, and Luca Salsi in Verdi’s “Rigoletto.”Ken Howard/Metropolitan OperaThe baritone Luca Salsi, in the title role, sounded firmer and less haunted than had Quinn Kelsey, with his echoey, indelibly wounded voice. The tenor Stephen Costello sang with blithe, poised arrogance — and his characteristic physical stiffness, his stock gestures, somehow worked. He became something of an automaton of power.When “Aida” — yet more Verdi — opened in the beginning of December, it was one of the season’s shakier efforts, with Latonia Moore struggling in the title role. Michelle Bradley had always been scheduled to take on the part in the new year, but her entrance was accelerated when Moore dropped out after the first performance.I returned on Dec. 27, and found Bradley in pleasant form: a demure, even reticent Aida. The tenor Brian Jagde was more nuanced, if also less steady, than he had been in his unrelenting first performance a few weeks earlier.The mezzo-soprano Olesya Petrova, who stayed on in the run when Anita Rachvelishvili, who was supposed to replace her, canceled after a single evening, also seemed more settled in. The most salutary change was Kelsey, who turned from Rigoletto to Amonasro, and who gave his trademark smoky tone and aura of threat to a role that, earlier in the month, George Gagnidze had rendered merely tight and querulous.In the machine that is the Met’s abridged holiday presentation of Mozart’s “The Magic Flute,” the baritone Benjamin Taylor, the second-cast Papageno, was the highlight on Dec. 28, his voice compact yet resonant, his charisma easygoing without being cloying. But for charm, even he couldn’t beat this “L’Elisir d’Amore.”Donizetti’s comedy is one of my favorite operas, but it can easily go awry. While laugh-out-loud funny, it is not a farce. Bartlett Sher’s quaint production interpolates a bit too much physical violence, presumably to raise the emotional stakes, but understands that the piece is at heart a small, sweet romance, drawing both smiles and tears.Thankfully, a cast led by the tenor Javier Camarena and the soprano Golda Schultz, and the spirited conductor Michele Gamba, in his Met debut, trust “Elisir” to reach the corners of the vast Met without overstatement or caricature.Camarena, as always, emanates sincerity and modesty; his Nemorino is a simple guy, but not a buffoon. After the slightest bit of burr to his top notes early on, they were pure and ringing by the end, and his “Una furtiva lagrima” began conversationally before breaking into golden rhapsody.Schultz’s tone had the gentle, silky glow of moonlight, but with a glisten that penetrated, and she gave a sense of both Adina’s independence and her vulnerability. The baritone Davide Luciano was suave as the conceited army sergeant Belcore; as the quack doctor Dulcamara, who provides the cheap wine that Nemorino takes as a love potion, the baritone Ambrogio Maestri was robust without being over-the-top.This was as lovely as opera gets. And it’s not over yet. After five more performances with this cast through January, “Elisir” comes back in April with Aleksandra Kurzak; the newcomers Xabier Anduaga and Jonah Hoskins; Joshua Hopkins; and Alex Esposito.Before that, in March, the soprano Angel Blue will star when “La Traviata” returns yet again. What will she add to Sierra’s and Jaho’s angles on the doomed, desperate Violetta? I know I’ll be there in the audience, ready to find yet more facets in these diamonds of the repertory. More