This is one of my favorite 'reworking the content' ideas, but also one which can go awry quickly if students get off target. A required 'rough draft' step might be a worthwhile inclusion.
The idea is explained in the directions, but may be varied as you see fit. I don't personally like just letting them write content-driven poetry from scratch, partly because it usually sucks, but also because one of the long-term skills I'm pushing is wrestling with word choice, being aware of meter and pacing and structure, etc. If you're in a Common Core state, you'll see those basic priorities in the ELA standards as well, but we don't talk about that in these here parts no more.
You should prepare a few examples using content you've already covered - something you just can't review often enough. If that's just not gonna happen, at least save good ones from year to year. You might also look at the master of this, Weird Al Yankovic - although he has limited content goals, whereas we're all about the substance, baby.
In case you have no idea what a 'Weird Al' is, he does 'parody' music and videos of hit songs. I'm too old to keep up with the new stuff, but here's a Nirvana song and video that was popular in the early 1990's. Obviously you're welcome to watch the whole thing, but at least give it through the first chorus (that's about 90 seconds for you old timers who'll complain you can't tell where the chorus is in all this blankity blank noise whateverhappenedtotheRighteousBrothersNowTHATwasmusic!) before going on to Weird Al's reworking of it:
And here's the Weird Al version. Note how he maintains the basic format, feel, structure, and rhyme scheme of the original work while changing the content and thus the message:
I'm telling you, this is a great assignment - but lay the groundwork well, teacher type person, or you'll be reading some serious doo-doo. I live to serve.
FOLLOW-UP: Several of you have asked about a more recent Weird Al video we were all excited to encounter this past year. In case you have a life and missed it, it's called "Word Crimes":
Yes, I enjoy this one. I don't use it for this activity partly because I despise Robin Thicke (the original artist of the song of which this is a parody) and partly because the original is a bit R-rated for use in school. That doesn't mean you can't - just don't, you know, get fired for showing skanky music videos in class and then blaming this website for the idea. I TOLD THEM TO USE SOMETHING WHOLESOME LIKE NIRVANA, MR. PRINCIPAL!