Many regular riders may not realize it, but the Washington Metro is the most distinctive subway system in the world. You can tour the New York City Subway, Paris’s Metro or London’s Underground and find stations that are more impressive than those in the District. But Metro is characterized by something the others lack: a distinctive, unified design.

Any station along the network is uniquely Metro: the soaring ceilings, the browns and oranges, the expanses of concrete. Brutalism, an architectural movement characterized by rough concrete and stark forms that influenced aspects of the system’s design, isn’t particularly popular right now. Neither is the 1970s-style of the brown and orange used in Metro stations and older cars. But modifying these elements, as the Washington Metropolitan Area Transit Authority is proposing, would be an irretrievable loss.

Architecture is most at risk right before it swings back into fashion. This has happened over and over. In the best cases, valuable original features are covered up with something else for a while, only to be restored later, sometimes at great expense. How many times have you heard of vinyl or carpet in a home being torn out to reveal hardwood or scalloping below? At worst, buildings are demolished to make way for the new, which is almost always blander than what was there before.

Remember how people used to think of mid-century design before “Mad Men”? Now Eames and Saarinen are evoked right alongside Frank Lloyd Wright and Daniel Burnham.

Yes, rough concrete is a harder sell than are the clean lines of glass and steel. It’s certainly possible that fashion won’t swing back to embrace ’70s design motifs, but I think it will: If skinny jeans can come back in vogue, anything can. Unlike skinny jeans, however, Metro stations and other constructions of the same era can’t be recreated easily. Once they’re gone, that’s it.

Metro General Manager Richard Sarles has said the makeover will usher in a “21st-century look,” but he should tread carefully. Stainless steel and cool colors are all the rage now, but like everything else, they will become dated. Instead, why not embrace what we have and make it better? By all means, increase lighting, improve safety and, better yet, add more trains. But don’t take out what makes Metro Metro.

The writer is an assistant professor of preservation planning at the University of Mary Washington.