Architecture & practice·Legacy systems and migration
the design was fine and half the work was making it coexist with what is already running.
Brownfield
Work constrained by an existing system that must keep working throughout. It is the normal condition of professional software, and it changes what a good design is: the winning option is often not the best end state but the one with a credible path from here. Estimating brownfield work as if it were greenfield is one of the most reliable ways to be badly wrong about a date.