Greetings and Salutations,
I am not an attorney. However, I am familiar with the laws of copyright, which is what you are trying to protect. Copyrighting your show is relatively straightforward. You submit the script of your show with all the proper paperwork, etc., and you have something to protect. It gets expensive after that, when you have to hire an attorney to enforce your copyright against some pilfering dog who was stolen your act or a substantial portion of it, as a copyright is only as good as your protection of it. So, unless your show is making you good sums of money, it's not likely to be worth keeping an attorney on retainer to keep the thieves at bay.
I know of a magician who has copyrighted a specific portion of his act, and he has and does send letters to performers who steal that portion of this show. I do not know if he has ever gone any further than sending the letter threatening legal action, but he has indicated that the letter has persuaded those errant performers that the bit was copyright protected and not a stock bit.
Would this be of assistance to you? I don't know. As always, the best advise is to contact an attorney, one familiar with copyright laws and their enforcement, and go from there. If that expense seems daunting, find the US copyright "office" on the web and find out how to submit material for copyright and submit it. When you find someone has stolen your show, send them a letter noting that the material is copyrighted and that legal action could be taken to protect the material. That may be enough. However, you may have to find that attorney, and then the question is, is it worth it? Only you can answer that question.
And while I'm not certain of my knowledge in this instance, I think if someone changes 10 percent or more of the copyrighted material, that material is now "different" enough to be considered someone else's property.
This, or a similar question, came up in discussion amongst a group of magicians. I pointed out that nearly anyone with minimal skills could steal my act. However, no one can steal my personality, which is what sells my act and makes it mine.
Just my $.02 (USD) worth. YMMV
Joe Zeman aka
The Mage Ulysses