Simply put, the Copyright Law of 1976 allows any songwriter, composer, arranger, or lyricist who assigned his or her work to a publisher from 1978 to the present to ask for his or her copyrights back 35 years after the work’s publication. Add 35 to 1978, and you get 2013.
So, if you published a song in 1978 and you’re not happy with what has been done with it, or, you’re not happy with how you are being compensated and you think you can do better, you can get your copyrights back, starting next year. You are due anytime between 2013 and 2018, the 40th year of the copyright term. Let it get to 2019, year 41, and you’re stuck – but now that you know better, why would you let that happen? Did your work get published in 1979? You can get your copyright back between 2014 and 2019. And so forth and so forth…
Now, here’s the thing – the Copyright Law of 1976 says you need to give your publisher a termination notice, and then there will be wrangling about when the song was actually published, and who the true author of the work was (if you wrote with other people and didn’t do some documents over who owns what part of the song, this will come up), and whether or not the record labels will try to say, if they first put your work out there, that your song is a “work for hire” and therefore not eligible… blah blah blah.