Obtaining a release for unused designs

Let me revise. Any advice on obtaining the IP for free? I realize this largely depends on who currently owns it. I have a great relationship with my former studio, and I don’t think it would be an issue there. Just some legal docs, I guess. Some of the past clients and brands I’ve worked for, however, have since left their companies, changed ownership, etc., and I no longer have a personal relationship with the new owners of the IP. How would you approach that situation?