I eventually learned to never trust any restrictions on the user.
I quickly learned to make sure everyone had a copy of decisions made, so that I could charge by the hour for changes. I eventually learned to include examples of what would and would not be possible in any specification or change order.
Also this change from 1:1 to 1:n entity was like one "minor" feature in a rather larger list of feature requests. It so far has caused more work then all the other features combined.