Rewriting, or paraphrasing, is a useful exercise when considering the meaning and the structure of a poem. First, sometimes when we put a poem into our own words in prose we gain a clearer and deeper understanding of the poem itself. This is always a benefit to you, the reader, and to the poet as s/he then has a greater and more appreciative audience. (Don't rewrite into poetry--that would be a different exercise altogether and might effectively come after you've rewritten, or paraphrased, the original poem.)
Second, the structure of poetry is so condensed because the words, phrases and thoughts are so precise. You will sometimes be amazed that three or four words of poetry require full prose sentences in an attempt to convey the same meaning. This proves rewriting (paraphrasing) to be a valuable exercise because you experience the power of precision wording for yourself, which gives you greater skill at mastering your language and its vocabulary.
A possible rewrite (paraphrase) of "Harlem" might be:
What happens to a dream that is delayed in coming true?
Does it dry up and wither like a grape in summer's heat?
Or does it become infected like a neglected sore?
Does it begin to carry a stench the way rotten meat does?
Or does it turn to sugary crust like fudge candy left out to turn to sugar?
Maybe a dream without fulfillment just sags from it's own weight like a heavy load.
Or does it simply explode from pent up despair?
I really like the previous answers and want to try my own hand at rewriting the poem. One of the main purposes of rewriting, I think, is to explore the structure and the full range of meanings of the words in the original. Ideally, too, there will be little repetition of the original words in the rewritten passage. So here goes:
When I don't get what I really want
What happens in me when I don't get what I really want?
Does what I really want start to fade from my mind, shink to almost nothing?
Or does it eat at me, make me ache inside?
Do I let it spoil my appetite?
Or do I swallow hard and try to forget it?
Perhaps it just makes me hunch over.
Or lash out in rage.
Your question isn't really clear, so I'm just shooting blind here, but I'll give it a shot. The Langston Hughes poem "Harlem," is well known and much referenced. The classic play, A Raisin in the Sun took inspiration from it. How could we rewrite it, and why would we choose to is an interestng question. Sometimes if writing poetry is difficult, we can get the creative juices flowing by mimicing the form of some well written poem such as "Harlem." The mimicing can be serious or a parody. Please keep in mind that as authors, we must give acknowledgement.