[Anechoic]

Imagining building a bridge and then in the middle someone comes along and says it should also be a tunnel

While converting a bridge to tunnel mid-construction doesn't happen, what does often happen is that design assumes a particular construction technique can be used, construction starts with that technique, and midway through it's determined that an entirely different technique is required. This results in a bunch of redesign, remobilization, etc. Just like with software, construction often does not survive first contact with reality.