The fire damper from the Jour Fixe
Agreed three years ago, not there today. To begin with that is not a construction question but the question of who promised what, and when.
What turns expensive about it
Liability comes first: who gave the commitment, who accepted it, and which deadlines are still running. Only after that does it become a matter of rectification and redesign, and in the bad case of the project dropping back into an earlier Leistungsphase, an earlier work stage under the HOAI, the German fee scale for architects and engineers.
Nobody at the table was there back then. The Bauherr, the client who is building, remembers it differently from the general contractor, and both remember honestly.