A small change in your graphics can produce a new look and big results. Your staff can probably use a little refreshing also. People get tired-looking, just like exhibits. Rotate you staff from time to time. Fresh faces create fresh interest.
Every audience is different.
The audience you see this year may change next year. Their needs may change, their ages will change, the gender make-up may change, the diversity will change. The only thing you can count on is change itself. Make sure you are adjusting your objective to take advantage of a changing audience. There is no set formula to help you calculate what is acceptable.