When we first release a book, we design a cover we think will sell and we send the book out into the world. Over time, though, we might find that the cover isn't as eye-catching as we wanted, or we've found something we like better.
Are we stuck with the former cover? No!
There's absolutely nothing wrong with changing out covers that don't work. In fact, that's one of the first things to look at if a book isn't selling.
Let me tell you a little story.
Back when I first started Trifecta, we released a book called The Princess and the Prom Queen by Jaclyn Weist. The cover concept was pretty straightforward - it's about two girls in two different eras of time, so we did a split cover with one girl on one side and one on the other, as you can see here:
This story is a retelling of Sleeping Beauty, and the author began getting fan mail asking for a sequel. She decided she wanted to use the evil sorceress more in the second book, and found a fantastic evil sorceress picture. This is how that cover came out:
All I have to do in order to change the cover is upload it to Kindle and CreateSpace. I don't have to get a new ISBN or anything because it's the same book - just in a different package.
So if you'd like to give an older book cover a little boost, feel free to experiment and see what you come up with. You're not obligated to keep the same cover forever if you'd like a change.