This seemed like a trivial change & when we brought it up, Derek & Bob both said it's not too big a change so ``no problem.''
Originally, the downstairs bathroom had a tub/shower combination. We thought about this (after the slab was poured & the walls framed) & decided to change it to a shower only. How much trouble could it be? Both require drains & water.
Surprise! A tub drain is smaller than a shower drain? Who knew? Since the drain had to be resized, two holes had to be cut into the slab. Oops. To me, cutting up the slab represents a big change.