In the future is can you notify all members that a change is goin to take place ? Maybe post a sticky to the top post of all forums. Or send out e-mails or do both. Explain in 24 - 48 hours the forum is going to change. Lock the post also so it won't get spamed or turn into a whine fest.
Here are some examples of the following- The Look (or view), Wso's , War Room,Headers, or Back Ground image, etc.
I know this site is corporate owned and will continue to make changes. If the members know ahead of time it may prevent some of the backlash. Or it may keep a few people from leaving.