I've read that a lot of writers will essentially sell the same article to different places, simply by tweaking the content. My question is, how much variation is required for you to sell the same article (with first rights) to more than one magazine?

To illustrate, I have an article I'm pitching to a national sewing magazine on how to sew doll clothes. Originally, the article was a three-part series, about 6,000 words long. To pitch it to the magazine, I condensed it to a single article, about 2,700 words. (This may still be too long, but I'll discuss that with the editor if/when they approve the idea.) Now, I have another magazine that I regularly write for, which specializes in historic costuming. I thought perhaps I could sell them the same article, provided I changed it enough. I would return to the original three-part series, and I would angle the subject matter so it focuses more on historic costumes, rather than sewing in general.

As I was revising the article, I realized there's a lot of content that, by its very nature, must overlap. The section about how to draft your own patterns, for example. I can rephrase sentences and adjust things so you're drafting historical costumes rather than modern doll clothes, but it's still the same basic idea. Is this okay? Basically, both articles would have the same bullet point outline, but I would tweak each one to be either modern or historical, based on which magazine I'm selling it to. Some paragraphs might essentially be saying the same thing, just rephrased. Is that okay?

I don't want to get in trouble for selling the same concept twice, so I really need to know where the line is drawn.