Sounds like you've already gotten some good advice.  I would suggest that you might want to make your Broker & Assistant tables date sensitive, i.e., with a StartDate and EndDate. 

This way you don't loose any data when an Assistant is promoted to a Broker status you just record the EndDate as an Assistant and his new Broker record will have a StartDate on the next day.

Now, if you want to keep a dated listing of all Broker/Assistant assignments you could do that in a separate table with constraints to the Broker & Assistant tables.  That way you can keep track of questions like "what Assistants did Bill have last year?" Or the fact that Jeff worked for Bill on three separate date ranges.

It all depends on where you want to go with all this power and knowledge.