The Constitution gives Congress the power to “promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries,” but what happens after that limited time is done--does that mean one can do whatever they want with something that has passed into the public domain?
For patents, the answer is yes. Once a patent expires it’s free game for anyone to take advantage of it without worrying about being sued by the inventor. Of course, there might be other considerations, certain inventions such as those deemed to be vital to National Security or those regulated by the FDA may still be illegal to make use of without additional clearances, but those are not concerns of patent law.
For copyright, the issue is a little more complicated. First, it’s not as easy as people might think to determine if something is really in the public domain. Just because something is old doesn’t mean it’s in the public domain. This chart gives a good breakdown of when copyright expires. The confusing mess of copyright duration is a prime example of what can happen when Congress tries to do. . . well, just about anything.
Second, certain works have “moral rights” attached to them. The US recognizes an authors right to attribution and integrity for works of visual art. These moral rights are separate from “usual” copyright protection.
So what, if anything is keeping me from taking Moby Dick, running a few search/replace commands in my word processor, and selling it as “Steve, His Friend Skippy, and the Big, Dumb Whale That Ate Steve’s Leg?” Other than being laughed at, not much.
Of course, everything is done online and is accessible by the entire world, which can further complicate things.
The US “moral rights” law does not extend as far as some other countries’ laws. In France, for example, I’d expect a problem with my version of Moby Dick. If French jurisdiction can reach me, I might be sued under their laws. There is also the possibility of a French judgment being entered against me and enforced in the US, even if I’m not under French jurisdiction. Generally, I think the possibility of that happening is close to zero because of the conflict that would create with our own Constitution, but it’s still something that should be at least considered.
Almost nothing is clear-cut in copyright law. I’d suggest that anyone wanting to play around in it get an opinion from an attorney that practices copyright law.