Is there a way for Yahoo to look up what IP address or person changed my password?
If the change was done from your computer, Yahoo will have a record of the same IP address, so no, they will not be able to know who changed anything if the person who changed it knew your user name and old password. If it was done at another computer with a different IP address, Yahoo might disclose to you the IP address, but that is technically invasion of privacy. So, it is not likely you can find out who changed your password. If you know it was done from your computer, you might be able to narrow it down to someone you know. That’s the best you can hope for. Always protect your account if using Yahoo!
