Quincy Mayor Thomas Koch, during a special event Tuesday meant to specifically address the downtown project, said he’s confident the $1.6 billion downtown redevelopment project will move forward, but said changes may need to be made to the land-disposition agreement between the city and Street-Works to make the project more affordable for the developer. The agreement, commonly referred to as an LDA, is the legal document signed in 2010 that establishes the terms and scope of both parties’ financial and development obligations.