"He who marches out of step hears another drum." ~ Ken Kesey
A long time ago a man named Roger Jennings taught me most of what I know about screen printing.
All true however making changes on the fly can bring some interesting challenges. Immediate changes can often cause disorder. Yes, many vendors and icons may come in and be persistent. The Boots on the Ground people sometimes know better. Not always, however they do it every day. I have most always found it best to get feedback from them