Whether you charge admission or not, if you use the specific work, you must have permission from the copyright holder. Just changing part of a play is not enough to avoid copyright violation. However, only the specific expression is protected. The underlying theme of any play, book, story, etc., can not be copyrighted. For example, compare "Romeo and Juliet" with "West Side Story." Both deal with star-crossed lovers, but they have very different settings, situations, language, and characters.
Answered on Oct 24th, 2013 at 10:33 PM