Branding is one of those things that's often misunderstood. A brand is the sum total of activities and assets that help you convey to the public what your firm does and its corresponding values. The better your brand, the more obvious it will be to an outsider what it is about.
However, a brand isn't something that you create and then forget, hoping that it will take care of itself. It needs to be constantly tended to, monitored and nurtured, so it can blossom into a mature brand someday.
I've seen businesses that have all the DNA of a great brand, but because they're not nurturing it, it ages and starts falling apart, to the point that it doesn't accurately represent the business anymore.
One of the more obvious elements of a brand, its visual identity, needs to evolve with time, both to stay current but also to reflect the internal changes that any business is going through. While a really good visual identity may never need to be redesigned, a regular refresh (every 5 to 10 years) is a good idea and common practice.
If your brand guidelines are from 1997, it's likely that it needs to be inspected and possibly, refreshed, so that your business continues to attract top clients. Many a corporate rebranding have gone awry in recent years, but a shining example of how to do it right was the recent rebranding of Burger King, which revived their 1970s branding, which is much better than its 1999 redesign, which was as bad as your typical sports team logo.