The article’s text said, “Winamp will remain the owner of the software.” That does not, in fact, preclude giving it a FOSS license, nor does retaining a related trademark. GP was correct. They can make it FOSS and keep the trademark and copyright. I don’t see any reason to think it unlikely.
It's possible. However, at no point in the post is that discussed, so it's pretty wild speculation.
Forking someone’s copyrighted work does not change ownership of the rights in any jurisdiction that I know of. If you meant “ownership” in a difference sense, like maybe control over a derivative project’s direction, then I think choosing a different word would have made your meaning more clear.
AFAIK, it doesn't "change" ownership, but it creates a new property with new ownership. That new ownership may be bound by he terms of the original license, but the original owner has no further control.