Can "public domain licensing" pull the rug under your feet?

I know this is better suited for law.SE and related places, but I wanted a less formal discussion.

I can slap a 2015-2025 GPL on previously unreleased code. If the code was already published without a license, I believe I can again slap a 2015-2025 GPL and, if confronted, argue that "I forgot" to add an explicit license notice previously. But can I slap a 2015-2025 GPL where I have previously had a 2015-2025 Unlicense (or any other reputable SPDX-listed license like WTFPL that tries to legally imitate the public domain)?

Both the Unlicense and CC0 are quite popular on GitHub. Can we rely on "public domain" software staying (at least) permissive? Or are we simply trusting the maintainers to not re-license everything "retroactively" in a patch release?

https://redd.it/1v6l4iv
@r_linux