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If there's an architectural version of the extreme makeover, it's the redo of four buildings in northeast San Francisco, where chunky concrete coats have been swapped for gowns of sleek, clear glass.

The buildings were built between 1963 and 1976, ungainly from the moment they appeared on the periphery of the downtown scene. Now they're in the middle of the action, old boxes in new wrapping meant to turn the heads of potential tenants and residents.

I'll cut to the chase: The repackaging hits the spot. Once-homely boxes have a fresh sheen. The renovations include changes on the ground that should connect the buildings to their surroundings in welcome new ways.

In a larger sense, though, the cultural aspects of the trend are more profound than the architectural ones. Beyond the current craze for glass, we're seeing how the map of a city evolves and how buildings, in turn, adapt. Along the way, what is saved and what is discarded bring the values of an era into focus.

The most obvious example of a building shedding its skin is at 100 Van Ness Ave., where a 32-story shaft served as the headquarters of the California State Automobile Association for three decades after its completion in 1976. Now, it's being reclad in floor-to-ceiling glass as part of a conversion into 400 or so apartments.

Another high-profile repackaging is 680 Folsom St., near Third Street. It debuted as a stout operations center for Pacific Telephone in 1963, perched atop black granite behind iron fencing. Today, the computer-card-like concrete facade is gone, and shallow bays have been added to break the 15-story monotony. The widest bay extends out to Folsom Street, helping frame a new podium-level plaza that meets the sidewalk with terraced stairs.

The other two cases, both nearing completion, are less visible from afar.

A former bank data center from 1974 at 155 Fifth St. soon will house University of the Pacific's Dugoni School of Dentistry, where seven stories of dark glass and drab concrete are newly wrapped in an institutional, but nonetheless more inviting, sheen. On 10th Street just south of Market, the 1975 annex to the Merchandise Mart - once covered in concrete panels stamped with countless M2 logos - has been streamlined with gray glass and a syncopated layer of metal blades "to add a little bit of texture" to the 10-story box, in the words of Terry Kwik at RMW, the architect for owner Shorenstein Properties.

While each project has its own story, the common thread is a 21st century emphasis on recycling. The skin goes, but the bones stay.

"We really tried to make a strong contrast with what had been there before," said Leo Chow at Skidmore, Owings & Merrill, the architecture firm at 680 Folsom St. "What we inherited was a cumbersome, unwelcoming mass."

But a robust one: Not only were few seismic upgrades required to the structural core, the change from heavy precast concrete to relatively light glass curtain walls allowed developer Boston Properties to add two stories of office space.

"We really like the idea of rehab and reuse - it's ecologically smart," Chow said.

That's the same sentiment voiced by Marianne O'Brien of SmithGroupJJR, the architect for the Dugoni School.

"Our strategy now with old buildings is to retain as much as possible of the existing structure," O'Brien said. "This one met code with only a few upgrades."

That case for large-scale reuse is similar to the argument made by historic preservationists who have worked to link their cause to the larger sustainability movement. But these stylistic reboots turn the idea behind the restoration of 19th century warehouses and industrial buildings on its head.

Consider: Those structures were workhorses that have earned our affection, revered for the immersive tactile joy of wooden timbers or load-bearing brick walls.

Not so with the desultory buildings of the modern era, the ones erected to do a job and little more. The buildings that have been reclad weren't misunderstood midcentury masterpieces: They embodied the bottom-line mentality of an era when urban cores were declining but technology didn't yet allow large employers to flee to the suburbs (or the 21st century equivalent, Bangalore and Bangladesh). Data centers and back offices were plopped down where land was cheap, designed to function smoothly while keeping out the riffraff.

Pride of place? Not part of the equation.

But the "good" parts of downtown San Francisco no longer stop at Union Square and Market Street. The just-finished 680 Folsom is almost fully leased. The former M2 at the corner of 10th and Stevenson streets shares the block with Twitter, which moved into the older mart structure (now dubbed Market Square). The economic rejuvenation of Mid-Market also explains the second life of 100 Van Ness as a residential address.

If the architectural upgrades are real, they don't change the fact that updated looks go only so far. The most elegant conversion is Skidmore's 680 Folsom St., taut tailored glass with metal accents on the west and north. It's still a squat slab with floor plates of more than an acre.

The real payoff, at least for the public at large, comes at sidewalk level. When 155 Fifth St. was a data center, for instance, it treated Fifth Street like an alley. Now there's a tall broad walkway notched into the base, breaking up the mass and acknowledging that pedestrians as well as automobiles might be passing by.

More than anything else, this quartet reminds us that architecture is not skin deep. There's only so much you can do for big buildings that were badly conceived.

The flip side is that a city is a work in progress where reinvention is part of the terrain. There's an ideal world and a pragmatic one - and pragmatically, partial fixes are better than no fixes at all.

John King is The San Francisco Chronicle's urban design critic. E-mail: jking@sfchronicle.com Twitter: @JohnKingSFChron

John King is The San Francisco Chronicle's urban design critic, taking stock of everything from Salesforce Tower to sea level rise and how the pandemic is redefining public space. A two-time Pulitzer Prize finalist and author of two books on San Francisco architecture, King joined The Chronicle in 1992 and covered City Hall before creating his current post. He is an honorary member of the American Society of Landscape Architects.