Some time ago, I wrote a small program to automate a simple task. At the time, I released the program under the GPL license. I'm the sole author of the work and it was not created for hire (I was not under employment contract while making it or being paid for it).
Now I've been contacted by someone who has reused and incorporated the work, who at the time did not notice it was licensed under GPL. I've been asked if I'd be willing to release the work under another license so they can keep using my work as part of a closed source commercial project. (Something they wrote at work).
I'm not against this, but am I even allowed to do that? Am I free to rescind the share-alike requirement of GPL by just issuing the same work under another, more permissible license?
Can I say "I release this to you under $license additionally?" I was thinking cc0 to completely wash my hands of it, but is that even possible? I don't want to expose myself to any liability doing this.