I agree with linney that there's no upgrade path from Ultimate to Enterpise. I don't think this will work because Enterprise has a slightly different feature set than any of the retail versions. However, you can change retail versions, by simply changing the product key, so it's worth a shot if you're going to make the change anyway. Obviously have your backup's, Easy Transfer, etc all completed before trying just in case something goes drastically wrong.
When you're ready to make the change, try going to Control Panel, System & Maintenance, System. Then down near the bottom, try changing the product key & see what happens.