Oooooh, that's a sticky one all right! The free patterns we receive on Knitty, etc. usually have a very concise copyright attached at the bottom of the page, don't they?
Well, in the art world...9 major changes must be incorporated into your artwork if you want to sell it as your own art design. Even then, some artists sue other artists over the issue...and sometimes win.
My company has dropped the images of certain artists like hot potatoes when another artist makes a claim of infringement. (and it is hard to copyright ART!) We just drop the artist rather than fuss and go to court over copyright laws. We expect the artists we carry to do their own homework prior to handing us their "new" work. If they get into hot water, we just drop their art.
It is good of you to think carefully about the issue. It shows that you are conscientious and honest-hearted.
A side-story: I make Thank You Cards, using rubber stamps. I have been approached numerous times about selling my cards to them, or in stores. I reply with: "The rubber stamps I used for these cards are copyrighted. I can't sell the cards. If you buy a piece of art from me, I will give you a thank-you card FOR FREE...but I can't sell my cards to you. You have to buy the stamp from the store, and make your own cards. That is how the original artist makes her money...if you buy the stamp!"
Anyway, read the copyrights closely.
IMHO: if you make enough changes to the item, it really becomes another design. I mean, how can one copyright "cables surrounded by rev st st"? Isn't that all the Irish Hiking Scarf is??