Does intellectual property licensing change the very moment the rightsholder makes a change? Or does it have to be officially conveyed in some way?

Suppose in 1997 after finishing writing the first Harry Potter book but before signing her first licensing or publishing deals, J.K. Rowling had written on a piece of paper "I hereby release all rights to all current and future Harry Potter books and their settings and characters to the public domain, signed J.K. Rowling." She then immediately burns the paper.

Legally, would that be enough to make it so that everything Harry Potter related is in the public domain from the moment she signed the paper, despite that fact that no one but her knows about it? Or would she have to officially file it somewhere for it to take effect?

If the truth later came out, like maybe there was a CCTV camera positioned in just the right position to see what she did, would it be enough to stand up in court?

And assuming it did, would licensees such as Warner Brothers have standing to sue over the sudden loss in value of their licenses?

In her particular case it would be subject to UK law, but for the purposes of this question let's assume that US law is what would control the situation.