And you keep the copyright notice, otherwise you're violating the license.
If it's based on the same design (i.e. same file formats, mechanisms, etc.) then the issue is still not giving credit and pointing out the design it's based on. Doesn't cost them a thing and gives a lot of goodwill from people.
Like, if someone uses my code, I'm happy, if they copy the idea and present it as their own that's dishonest.