At least one and possibly all five towers at the massive Riverside Center development will not be the work of Pritzker Prize-winning architect Christian de Portzamparc. The French designer helped Extell Development and the Carlyle Group sell their swank plans' to the community and the City Planning Commission. The latter was so taken with the crystalline designs of Mr. de Portzamparc, who also designed the LVMH headquarters and Extell's One57 tower, that restrictive zoning covenants were set to ensure the buildings would look as promised.

But now, Extell and Carlyle have turned over one of their tower sites to the Dermot Company, which has hired local firm SLCE to design the apartment building on the West End Avenue section of the site. While Dermot insists its project will be up to the standards promised during last year's public review process, some, including the exacting City Planning chair Amanda Burden, worry the design doppelgangers will lead to lesser work.

“I am extremely disappointed to learn that the developer of Riverside Center has chosen not to retain Christian de Portzamparc as architect for this project," Ms. Burden said in a statement.

When Dermot came to the local community board last month to present its version of the designs, there was some disappointment that they had not been joined by Mr. de Portzamparc. "If you look at it, they're more usual, they've probably been value-engineered," Ehtel Shefer, chair of the board's Riverside Center working group, told The Observer in a phone interview. "I don't know if it's the feeling of the entire board, but certainly some people were disappointed."

Back in 2005, Carlyle and Extell bought the remaining undeveloped portion of Donald Trump's Riverside South development from his Hong Kong partners (to the consternation of Mr. Trump) for $1.76 million. Much of it has since been developed as new towers by Gary Barnett, Extell's principal, but the southernmost parcel had to be rezoned because previous plans called for a new television studio to be built on the site.

Instead, Mr. Barnett trotted out his plan for a 3.1-million-square-foot city within a city within a city designed by Mr. de Portzamparc. Five jagged towers were arrayed around three acres of open space. After much back-and-forth with Councilwoman Gail Brewer, the developers agreed to building 20 percent of the apartments as affordable housing and to include a school on the site.

When it came time to start building, Carlyle, which controls a majority stake in the site, decided to hold a competitive bidding process, to which Extell was invited but not guaranteed the chance to build the first tower. Instead, the prize went to Dermot. When it comes time to build the remaining four parcels, Carlyle expects to go through the same private bidding process.

Mr. Barnett said that given the large amount of affordable housing and the school in the first building, he was less interested in winning the project. He still hopes to take the lead on some, if not all, of the other development sites, though he acknowledged there was no guarantee any of the towers would be his to build.

"I hope we get to build some, but I don't know," he said. "If we do, I can tell you, Christian de Portzamparc will be our architect."

Carlyle declined to comment.

Councilwoman Brewer was ambivalent about the changes. "I was more concerned with the school and the affordable housing, but I can see why people might be angry about this," she said, adding that of Mr. de Portzamparc, "They certainly made a hard sell for him during the ULURP."

In a brief statement, Dermot principal and COO Stephen Benjamin, stressed that his tower was still under design and, given the zoning covenants, would still resemble what was originally proposed. “We are in the midst of the design process for a spectacular building that will be in full compliance with the zoning as is our obligation and right," Mr. Benjamin said.

Ms. Burden raised the same point in her statement, that even if the de Portzamparc name is not on the final buildings, his master plan for the site remains, and the essence of his work will persist.

"The integrity of de Portzamparc’s work will be maintained because key architectural features—including, among others, building silhouettes, distinctive sloped and angled sculptural  forms, facets and sloping tower tops—are embodied in the land use approval and are a condition of developing the site," Ms. Burden said. "De Portzamparc was instrumental not only in shaping the site but also in developing these design controls."

"The City Planning Commission fully understands that a developer may decide to change architects over time for a number of reasons," she continued. "De Portzamparc’s important contribution to this project will survive this developer’s decision to look elsewhere for design services."

This is not to suggest that SLCE is necessarily a worse choice as the project's architects, either. In fact, the firm is frequently brought on by developers to serve as the architect of record for more highfalutin designers like Mr. de Portzamparc.

The globetrotting architects (Gehry, Nouvel,Koolhaas and the rest of the designer jetset) are not usually experienced in the intricacies of jurisdictional buildings codes, idealized layouts and local tastes. It falls to firms like SLCE and Goldstein Hill & West—which has also done extensive work at Riverside Center and may well wind up designing some of these tower—to bring a strarchitect's dream into the realm of the buildable, the inhabitable, the comfortable.

That said, some connoisseur's counter that these firms' work can be pedestrian and developer-driven, lacking the flair of some of their more renowned rivals.

Neither is this switcheroo exactly new. That is a big part of the reason the City Planning Commission works so hard to ensure certain design flourishes and details in ambitious projects like the Riverside Center.

Mexican master Enrique Norten was originally pitched as the architect of the Edge condominium towers on the Williamsburg waterfront, only to be swapped out for Stephen P. Jacobs and Associates when it came time to build. Richard Meier and SOM came up with the scheme for Sheldon Solow's Con Edison development just south of the United Nations. Parcels have since been sold off, with more on the block, and it is uncertain who might wind up conceiving of the final projects.

Most famously, Bruce Ratner dumped Frank Gehry from his Atlantic Yards project after the recession led to a reevalution of the work of the Pritzker Prize winner, whose buildings are notoriously expensive and difficult to execute. Mr. Ratner tapped Ellerbe Beckett, a no-nonsense designer of sports venues throughout the nation, to replace Mr. Gehry on the Barclays Center. Even though she had no oversight of the project at any point, Ms. Burden was said to be so bothered by the switch that she successfully lobbied to have local wunderkinds SHoP brought on to help redesign the facilities.

The most apt comparison to the situation at Riverside Center might be what happened at the World Trade Center. Little remains of Daniel Libeksind's masterplan but the outlines of the project, a crescent of towers stepping down in size along Greenwich Street. Granted, some of the world's most famous architects have stepped in to replace Mr. Libeskind. At the same time, the strict design controls at Riverside Center will make any successors who are not Mr. de Portzamparc stick much closer to the French architect's vision.

Ms. Sheffer points to an incident closer to home, Fordham University. The school was working on plans to redevelop its Manhattan campus near Lincoln Center, which included selling off some parcels for development. While there was a general consensus that a residential tower proposed by Douglaston Development was too big, people at least seemed to like the design created by celebrated architect Cesar Pelli, he of World Financial Center and Bloomberg tower fame.

"The deal seemed to be the deal," Ms. Sheffer said of Fordham's plan, "but then they made a deal with Glenwood, and suddenly it looks like everything else Glenwood has done and that's not every interesting."

She hopes this will not be the case at Riverside Center. "I think the feeling was, the de Portzamparc buildings, it was a vision, a particular kind of towers that related to each other and to the open space and the neighborhood, with the view down to the river," Ms. Sheffer said. "In the renderings, it certainly didn't look like the rest of Riverside South, and that was a good thing. But now, we can't be so sure. Maybe it's a lot of hype, maybe not."

But would this be New York City real estate if that were not the case?

Update: This story has been changed to include details about other projects that have gone through similar design switches as well as other projects SLCE has worked on. Also, a previous version stated that Dermot was developing two towers on the site, rather than just one. The Observer regrets the error.