Short answer: You can do what you want.
If you are the original author of a work, you have the right to do anything with it. Whether you gave it away under some license before doesn't matter. You can release it under a different license, if you want to, whether you modify it or not. If you released it under some "cannot sell" license you could still sell it. The license you gave other people with their copy doesn't affect you, only the people who got it from you with that license.
Additionally, if something was released into the public domain, that means that anyone is allowed to do anything (including all of the things above), not just the original author.
 Unless it was written as a "work for hire" and the contract had specific legal restrictions to the contrary.