Here's the contract language from a few of the large play publishers stating their policy on script changes:

Samuel French
The play will be presented as it appears in published form and the author’s intent will be respected in production. No changes, interpolations, or deletions in the text, lyrics, music, title or gender of the characters shall be made for the purpose of production. This includes changes or updating the time and place/setting of the play. In reference to changing the gender of characters men, will play male roles and women will play female roles. Please note: each title is considered separately and whenever you wish to make changes to a script you must always request permission in writing. Not all authors/authors’ representatives allow changes to be made.

Dramatists Play Service
The play(s) must be presented only as published in the Dramatists Play Service, Inc. authorized acting edition, without any changes, additions, alterations or deletions to the text and title(s). These restrictions shall include, without limitation, not altering, updating or amending the time, locales or settings of the play(s) in any way. The gender of the characters shall also not be changed or altered in any way, e.g., by costume or physical change.

Music Theatre International
When you are granted a performance license, by law the show you license must be performed “as is.” You have no right to make any changes at all unless you have obtained prior written permission from us to do so. Otherwise, any changes violate the authors’ rights under federal copyright law. Without prior permission from MTI, your actions may subject you to liability - not only to the authors, but also to us-- for breaching the terms of your license agreement, which clearly forbids you to make any changes or deletions. Occasionally, new versions of shows are created when the authors or someone the authors have approved reconceives the piece. However, only the authors have the right to make these revisions and they rarely grant third parties permission to do so. If you feel you must experiment with reconceiving a show, there are many already in the public domain (Shakespeare, Gilbert and Sullivan) which are no longer protected by U.S. copyright law.