“Candide” in an opera house. “Spamalot” and “Rent” cheek by jowl with Shakespeare. But treating them as classics may not be doing them justice.There comes a moment in the afterlife of even the most successful musical when it threatens to become a museum piece. One day, it’s the hot new thing, perhaps even defining its era; next, it’s “The Merry Widow.”On a theater trip through New York State and Ontario last month, I saw three musical revivals in various stages of that transformation: one — “Candide” — fully evolved into an opera house staple that’s rarely performed anywhere else; the other two — “Rent” and “Spamalot” — on uncertain trajectories toward classic status or the dustbin.The “Candide,” at the Glimmerglass Festival in Cooperstown, N.Y., opened with what seemed to be an acknowledgment of the situation. From a stageful of shadows at the Alice Busch Opera Theater, Glimmerglass’s home on sparkling Otsego Lake, dim forms awakened as if from a long slumber, emerging from tarps and storage trunks. Eventually a sort of ghostly maitre d’ cued the orchestra, which sprang to life with the undying joy of Leonard Bernstein’s overture.It was an indication that the somewhat zombified story of “Candide” would always need resuscitating by the music. Rejiggered every which way since it was first produced on Broadway in 1956, the book has so many problems and variations that the options for reviving it resemble a game of 3-D chess. And the list of musical numbers Bernstein wrote to accommodate the changes — then discarded, rewrote, re-discarded, recombined and otherwise cycled into and out of the score — comes to nearly 100 titles.Glimmerglass’s version, originally produced there in 2015, is itself a revival, no more dramaturgically coherent in Francesca Zambello’s staging than any other. Though adapted from a Voltaire novella generally considered a masterpiece, its story — an innocent boy’s education in optimism is undone by the ever more absurd horrors of the actual world — becomes a case of diminishing returns when staged. Nora Ephron, noting that you get tired of the characters’ misadventures long before they do, called it a musical that always seems to be great “on the night you’re not seeing it.”True, yet it is at the same time glorious. Young singers with clarion voices — and a 42-piece opera orchestra, conducted with incisive good humor by Joseph Colaneri — bear you swiftly through the longueurs. In the process, a flop that tried too hard to be au courant, satirizing America’s postwar euphoria, is transformed into a timeless piece that, having found its niche, lives on and on. When Candide and his lover, separated by various disasters, sing the lovely and witty “You Were Dead, You Know,” they might be singing about the show itself.Brian Vu, center, as the title character in “Candide,” whose optimism is unraveled over the course of the musical by the horrors of the real world.Evan Zimmerman/The Glimmerglass FestivalThere’s a similar moment in “Spamalot,” the deliberately ludicrous musical by Eric Idle and the composer John Du Prez. If you’re familiar with “Monty Python and the Holy Grail,” the 1975 movie on which it’s closely based, you’ll probably be laughing even before a chorus of medieval plague victims, being carted off in tumbrels, sing pathetically that they’re “not dead yet.” One of them insists he’s in fact feeling much better.I don’t know whether “Spamalot,” playing this year at the Stratford Festival in Stratford, Ontario, can expect a similar recovery. A Broadway hit in 2005, it offered silly distraction and precision direction (by no less than Mike Nichols) in the ongoing dark after the Sept. 11 attacks. Not that it was designed to speak to its time, let alone all time; it was content just to fill time. Its ambitions seemed limited to rhyming “Lancelot” with “dance a lot” and trotting out a Python dream team including the French taunter, the knights who say “Ni” and a chorus line of self-flagellating monks bonking themselves on the upbeat.Like the movie, it was a blast, even if its satire, coming from all directions, seemed to have no target. (Much of what it pokes fun at are the conventions of musicals themselves.) Seeing it at Stratford, as part of a 12-show repertory that includes four Shakespeare productions as well as new plays and modern classics, is a disorienting experience. As comedies go, it’s no “Much Ado About Nothing.” The festival’s dignity and its ethos of highbrow good work do something weird to material so deliberately lowbrow and anti-establishment.In 2018 at Stratford, “The Rocky Horror Show” suffered from a similar problem — but recent Stratford productions of “Chicago,” “The Music Man” and “Guys and Dolls” (all directed and choreographed by Donna Feore) did not. The festival does sincerity, even the gimlet-eyed kind, very well. But as directed by Lezlie Wade and choreographed by Jesse Robb, “Spamalot” feels hasty and mechanical, relying on the prefab jokes to do most of the work. They don’t.Yet it’s not clear to me that even a fresher and more idiomatic take would solve much. (We’ll have a chance to find out with the arrival of a completely different “Spamalot” revival on Broadway this fall.) For many of us, the punchlines are so ingrained that they have become golden oldies, suitable for a kind of karaoke pleasure but unlikely to produce helpless guffaws. Maybe comedy needs to skip a few generations until minds that know nothing of migratory coconuts can test its enduring worth.But what about tragedy? For the sake of argument, let’s call “Rent” a tragedy even though it does everything in its considerable power to turn the nightmare of AIDS in the late 1980s, recalling parallel plagues in its 19th-century sources, into musical theater uplift. And time has further distorted it. In the manner of “Climb Ev’ry Mountain” and “You’ll Never Walk Alone” in their time, the show’s big anthem, “Seasons of Love,” has now delaminated from its story entirely. Instead of a plea to treasure brief lives, it has become an all-purpose good-times chorale; my sons (today in their late 20s) sang it at their elementary school graduations.Nestor Lozano Jr., center, as the drag queen Angel in the 2023 Stratford Festival production of “Rent.”David HouAn author should be so lucky as to have that problem, but it nevertheless is a problem. So is the meta-tragedy surrounding “Rent,” whose author, Jonathan Larson, died at 35 in the hours just before the show’s scheduled premiere. The work has essentially been frozen as he left it that day in 1996. Thom Allison, who directed Stratford’s production, told me that permission for even the tiniest change in the script, to correct an acknowledged inconsistency, was denied by the estate’s representatives.That leaves new generations little wiggle room in which to experiment with refreshing “Rent” and finessing its headaches. As always, it struck me in the Stratford production that the work of the downtown artists the show means to valorize is actually terrible; that the central male character is utterly passive; that its credibility as history is all but shattered by the last-minute resurrection of a character we’ve just watched succumb to AIDS. Having seen “Spamalot” the night before, I was surprised she didn’t sing “I’m Not Dead Yet” as she awoke.Yet Allison’s staging at Stratford’s flagship Festival Theater, also home this season to “Much Ado About Nothing” and “King Lear,” made a pretty good case that, in its scale at least, “Rent” can hold its own in such company. Certainly the story of the drag queen Angel and her lover Tom Collins (traced in the songs “Today 4 U,” “I’ll Cover You” and “Santa Fe”) has a full arc and tragic grandeur, enhanced here by frankness. The sight of Angel, beneath her drag, covered in Kaposi’s sarcoma lesions from neck to ankle (thanks to a cleverly made body suit) sent me reeling back to 1989.The question is whether “Rent” can be meaningful even for those unable to be reeled back that way. The Stratford production makes the case that it can, but however much the appearance of a new section of the AIDS quilt during the finale moved me, I wondered how many people under 40 even knew what it was. Some shows are so of their moment that they cannot be wholly of ours.CandideThrough Aug. 20 at the Glimmerglass Festival, Cooperstown, N.Y.; glimmerglass.org. Running time: 2 hours 45 minutes.SpamalotThrough Oct. 28 at the Stratford Festival, Stratford, Ontario; stratfordfestival.ca. Running time: 2 hours 15 minutes.RentThrough Oct. 28 at the Stratford Festival, Stratford, Ontario; stratfordfestival.ca. Running time: 2 hours 41 minutes. More