I once was in a similar situation, and had all of my design and stuff ripped from my version of the site, to be thrown into a terrible table design. All I can do now is laugh and say "I told you so".
WHen the switch initially happened, I hadn't been paid a dime for any work I was working on. So there were all my photos, graphics, code, and other such stuff. When I demanded that they remove the design and everything, they refused. I brought up copyright law, and they still refused. I got a pro-bono lawyer involved and told them I've got a lawyer, and they're going down. I took him to a meeting and when he explained what exactly the copyright law they were breaking (artistic) then they immediately offered me money for my time to purchase the images off of me. I took the deal as I didn't want to exacerbate the issue further.
What did I learn? Get the money up front. THen, if you need to, refund them payment or give them a credit. In certain situations, bill for more money.
Sometimes project planning doesn't go far enough. Sometimes, it may be good for you to start using UML designs to figure out just how much work will go into the project, and approximately how much they will be billed.