Ordinarily, one would cheer heartily for the decision that was announced Tuesday morning at the Martin Luther King Jr. Memorial Library: The architectural team of Mecanoo and Martinez + Johnson has been chosen to renovate and perhaps enlarge the historic but troubled main branch of the city’s library. Mecanoo, a Dutch-based firm, will play the lead design role, and it will bring to a city of buttoned-down corporate and institutional architecture a stylish European aesthetic that could have a major impact on the city’s downtown core.

But the MLK library is no ordinary building, and this will be no ordinary project. Opened in 1972, the library was designed by Mies van der Rohe, in his signature austere and rectilinear style, a solemn dark box of steel and glass with one bold message: Transparency is essential to democracy, as books are essential to civilization.

The building on Ninth and G streets NW was never perfect, and it’s long been overdue for renovation. But the choice of Mecanoo suggests there may be complicated and painful process ahead, as concerns about historic preservation clash with the library’s urgent desire for a more functional, welcoming and contemporary building.

Among the three finalists, which presented their ideas on Friday and Saturday to a committee of library, planning and preservation experts, another proposal, by Patkau and Ayers Saint Gross, was the more obvious choice if preservation is the central concern. Mecanoo, along with the team of Studios Architecture and the Freelon Group, presented much bolder solutions to the building’s problems, including the possible addition of residential units on the roof in an oblique addition crossing the library’s floor plan on a diagonal.

But former chief librarian Ginnie Cooper, who chaired the committee that made the decision, said that Tuesday’s choice was not a choice of a design, merely the selection of an architect. Much will change as the chosen team grapples with the realities of the building, the preservation-oversight process, the library’s desires and the community’s needs. She said elements from all three proposals may eventually find their way into the final design.

Cooper, who retired last year, oversaw a historic program of renovation and revitalization of the city’s libraries that brought major improvements or entirely new construction of 15 neighborhood libraries. Her leadership probably played a major role in D.C. Mayor Vincent C. Gray’s commitment of $103 million toward renovating the MLK building.

“I was here almost eight years, and I never liked this building,” she said. But as the library began making improvements, including retouching the main floor space in a way that burnished Mies’s design, she said everyone began to see possibilities in the old structure.

“There is every chance that this building can be the wonderful library we all want it to be,” she said.

The proposal submitted by Mecanoo and Martinez + Johnson stresses openness and environmental sustainability. It would enliven the library’s main floor by removing many of the brick walls that Mies used to heighten the geometric contrast between transparency and solidity. Large, glass staircases would connect the main floor to spaces above, opening up the box-like atrium, which often feels more like a waiting room or empty lounge, disconnected from the reading and research spaces on higher floors. A book store and cafe would be added to one of the original reading rooms, and by cutting through the sidewalk on G Street NW, light would flow into reconfigured space below grade. An “informal” auditorium would be added, and two large digital screens would greet visitors as they enter the great hall, now dubbed the “Marketplace.”

Another more traditional auditorium would be added on the fourth floor, along with the “Mies Restaurant,” which already sounds like an increasingly common game of memory and loss, in which businesses brand the very thing their presence replaces.

The biggest changes would come on top of the building. If the city decides to add space to the library — which could help raise revenue toward the final cost of the project, estimated at $225 million to $250 million — the architects propose two parallel lines of apartments, joined by a central atrium, set above a terrace garden open to the public. But that could change, as the city considers other possible uses for new space, including offices.

The new residential units proposed by the design team would raise the height of the building by three floors, and renderings suggest a floating volume that looks as if it has been slightly twisted away from alignment with the city’s grid system. This may pay homage to the angles created by the avenues in Pierre Charles L’Enfant’s original plan for the city, or it may just look a little screwy. If the city chooses to add space to the library — the decision will come later this spring — it will be a formidable challenge to make it work without the new volume seeming to sit like a hat, or an unwanted spaceship landing on the indifferent bulk of Mies’s original building.

The key player in the decision announced Tuesday may be Martinez + Johnson, a D.C.-based firm with particular expertise in renovation and historic preservation projects. It will be the local custodian of the team’s ideas as the project moves forward and faces review by the various city and federal groups, including the Historic Preservation Review Board, the U.S. Commission of Fine Arts and the National Capital Planning Commission.

They will also have to advocate for the design to the people of Washington, and that is always a complex and often frustrating process. At Tuesday’s announcement, activist Chris Otten held up a sign calling for a citizens task force to be part of the discussion. Otten feels the decision process, which invited public input, was still not open enough. Now that an architectural team has been chosen, he says he feels decisions about the building’s program should include greater citizen input, including design charettes.

“An online discussion board isn’t going to do it,” Otten said.

The prospect of selling their ideas to the public was clearly on the mind of Tom Johnson, principal at Martinez + Johnson, who said at the news conference, “In [Martin Luther King Jr.’s] honor, we will strive to create a library of the people.”

Maintaining the integrity of Mies’s design will almost certainly be the most fraught part of that process. Since the MLK main library was opened, libraries have evolved away from Mies’s central message — transparency and learning — to become community centers, meeting places, day-care facilities, computer lounges and many other things that Mies never envisaged. All of those new services will have advocates in the community who value and need them; the advocates for Mies’s design will include historic preservation experts, architecture critics and other cranks, who ground their beliefs on the complicated and difficult to express fundamental value of retaining important cultural objects simply because they are beautiful or play an important role in the history of culture.

Which is to say, arguing for everything that is flashy and innovative in Mecanoo’s plan will be far easier than arguing for Mies van der Rohe, whose modernist puritanism no longer seems quite so democratic as it did in the 1960s and ’70s.

Ordinarily, in this town of dull, boxy, uninspired architecture, boldness should be rewarded, and Tuesday’s decision is a bold one. Preservationists will have a lot of work ahead of them in the next few years, if the D.C. Council approves the contract offered to the winning design team.

But now, with a team chosen and momentum building for big changes, it’s hard to be sure of the greater danger: That too much of Mies will be lost, or too many of the interesting aspects of the design team’s proposal will be watered down.