My dogs are chipped with different companies. I went online to update my info after I got Taggart from the shelter and after I got Zoee done. I was able to add pictures to the page as well.
I believe they can tell you what "brand" of chip you have when they scan at the vet. When I had them check Taggarts she asked "HomeAgain?" and that's what his is. I haven't had Zoee's checked yet but might do that tomorrow. I'll let you know if they can tell what company, then you could just go online. Although they might need the number. There should be some kind of security so not just anyone can change info though. The previous owner probably should have changed it to your name.
I'll ask when I get Zoee's checked though and let you know what I find out. If someone else doesn't have an answer for you before that.