Just two months before the Museum of Contemporary Art San Diego is scheduled to begin demolition on a portion of its La Jolla building to make way for an expansion and renovation, more than seven dozen critics, architects and architectural historians have signed an open letter describing some of the proposed changes as “a tremendous mistake.”

The $75-million expansion, designed by New York-based Selldorf Architects, is set to add 30,000 square feet of gallery space to the museum, which has lacked a dedicated area in which to show its permanent collection. As part of the plan, the museum’s entrance will be moved south along the museum’s Prospect Street facade.

The entryway, completed in 1996, was designed in the Postmodern style by the influential architects Robert Venturi and Denise Scott Brown. (He is a Pritzker Prize-winner, and many believe she should have received the honor as well.) The design from their Philadelphia firm Venturi Scott Brown includes an exterior colonnade that leads into Axline Court, a neon-accented atrium topped by a star-shaped clerestory window. The atrium serves as the museum’s principal point of access. Under the renovation plan, the colonnade and the entrance will be removed and the atrium will be repurposed as a public gathering space.