At this stage in the production I'd be surprised if even 50% of the total shots were final in lighting (so totally finished renders) and the shots which are final will likely be the simpler shots. So in practice a model/character update now would be manageable if, and it's a big if, the director can make a quick decision about what he wants. The tricky thing is how much animation can be salvaged from the old rig/character and reapplied to the new one. If the face changes significantly then a lot of the close up and lip sync shots will have to be redone. Also this will cause a massive headache for CFX as all of their work will have to be thrown away and redone, if they work smart that may not be too bad but knowing MPC I suspect they haven't.

TL;DR

If the director can make a quick decision and the design change is minimal then everything should be fine