Jed,

technically, a person DOES need your permission to perform your songs in public...but by joining a PRO (ASCAP, BMI, etc), you assign the PRO the right to give that permission and collect the requisite fee for you.

Dick, that's not quite true. Copyright has to do, literally, with the right to make copies. Permissions and payment are just the mechanics of how those rights are administered.

There is indeed something to stop you from changing a song: in accepting a compulsory license to record a song, you agree not to change the song's "fundamental character." You also agree that your work does not qualify as a derivative work to be copyrighted itself. If a songwriter felt that you HAD altered the song's fundamental character, he could sue you for infringement...but it would cost everyone money and do no one any good, so this rarely happens.   

It's unclear what you're saying about Dylan, but whatever it is I think you've misunderstood the law. It's not true that "if it's changed enough you can copyright it again."

What Dylan did when using traditional material was to copyright his own songs based on the fact that the traditional material was not subject to copyright because it was in the Public Domain. So only the parts he wrote are protected, not the traditional parts; they are by definition excluded from his copyright.

If Dylan copyrighted the same song twice in two different versions (is this what you're saying?) then the second is a "derivative work," and as such the second copyright only protects the new bits. The older bits are still protected by the copyright on the first version, and their protection will expire when it expires. This is an important distinction, because otherwise you could extend the length of copyright on songs indefinitely just by adding more verses and "copyrighting all over again."