When it comes to what is allowed for pro/am competitions, it's often a good idea to check with the competition organizers because things vary so much.
When I was doing Pro/Am, "Open Bronze" meant that you could dance Bronze steps and some Silver steps, and so that category was considered a "bridge" between Bronze and Silver. It was still divided up by sex and age sub-category, just like the rest of the Pro/Am syllabus events.