earlier entry on the subject).
Who owns what? Assuming you're not an employee of the nonprofit, and assuming you didn't sign any paperwork assigning your rights in the articles, you own the copyright and can reuse the material however you like. Most likely, what happened is that you gave the nonprofit an implied nonexclusive license to use the article and that's about all. If you did execute an agreement with the nonprofits, (1) review the rules on works made for hire to determine whether your agreement qualifies as work made for hire; and (2) review the rules regarding assignments to determine if you have assigned your rights. Our guess is that you haven't done either.
Note to nonprofits. If you run a nonprofit and want to acquire rights from freelancers, we suggest that you use a work made for hire agreement, license, or assignment. You can put together a simple agreement with the aid of an attorney or by using our permissions book. And speaking of books and nonprofits, may we also recommend one of our favorites on nonprofit fundraising.