Taylor Swift sold the rights to six albums of her work (or was talked out of those rights, whichever), then went out and recorded four* of those same six albums again specifically in order to recover those rights, with the other two* yet to come.
No, she didn’t own the recording masters for those albums. She still owned the compositions, but not the recordings that were used to make those albums. By re-recording them, she did not regain the rights to the original masters and can’t stop them from being sold by their current owner. She has effectively supplanted them with new versions that she owns and markets as superior versions.
Edit: I love that she did this, but note that only Taylor Swift and maybe a few other artists have the fan muscle and financial resources to pull something like this off at an appreciable scale. But yeah, she didn't violate earlier copyright agreements regarding her masters (not the rights to the compositions themselves), she just did an incredible end run around a crappy contract.
So you can still go on, say, iTunes and buy "Shake It Off" (the original version which is not owned by Swift but for which she presumably receives a royalty), OR you can buy "Shake It Off (Taylor's Version)", which she/her business owns, and is the one that she wants you to buy. The point is that she still can't stop the original from being sold; she has not recovered those rights.
Edit 2: But her move
did effectively recover her right to perform her music publicly, since now she can say that she is performing the versions that she owns, and not ones that Big Machine Records owns. Kudos!
Edit 3: now I am wondering if she has to do some sort of disclaimer before her live shows, to the effect that everything she performs from those first albums is based on the "Taylor's Versions" re-recordings.
It is crucial always to consult an attorney before you sign any contract, so that if you have “Bad Blood” with your prior record label...
www.flblaw.com
So if I write a poem (or a D&D module!), and then write another one that's very similar to the first and may or may not even be based on it, I've plagiarized myself. My point is this is (or certainly should be) both legal and acceptable....even if self-defeating in the long run as people get tired of reading the same old stuff.
Plagiarizing yourself and committing copyright infringement against work that you sold are different things. It definitely would not, and should not, be legally acceptable for you to turn around and sell the exact same poem, or substantial pieces of it, to another publisher if you signed an exclusive deal with the first or sold it to them outright.
If you wrote another poem that was similar because that’s the way you write poems, you’re probably fine, as the Fogerty case illustrates.
For example, as a grad student, I was hired to design an educational board game. I signed a contract essentially ceding all my rights to it to the publisher. I cannot now republish it or substantial parts of it for profit, or I would be infringing their copyright. But I can design and publish other games that reuse or expand my ideas, as long as I’m not crossing that line.
Edit: Plagiarizing yourself and committing copyright infringement
can be connected, however, which is why students are warned that copyright infringement is one potential consequence of self-plagiarism (if you are doing it with published work). But mostly self-plagiarism is wrong because it is seen as dishonest, not legally actionable. Almost 100% of the time it consists of students reusing work that has already been used for another assessment, without acknowledging it as such (trying to hand in the same term paper twice, for example).