Take a look at Pepsi. They've been around since 1898. Every few years they've tweaked their logo slightly to stay relevant in the ever changing market place.
From 1898 to today, the logos are drastically different, each perfectly representing the era they were developed for, but when looking at the progression it's easy to see how each grew into the next.
So if you're current logo is starting to look a little rusty, don't toss her aside forgetting all the good years you've had together and look for a newer model. Remember all the history you have together, and consider a tune up and fresh coat of paint. Your logo and your business will thank you!