That question comes down to what's in your original agreement, and in particular whether (and how) 'derivative works' were included or excluded. You might want a lawyer to look it over.
Here's my non-lawyer view: In the most restrictive case, where you sold everything as a "work for hire" and agreed not to create derivative works, you still have the right to create a new similar app to sell, but only if you use none of the code and images and development work used for the original app. Start entirely from scratch. That's basically how the Compaq computer was created as a clone for the IBM PC, but Compaq took an extra step: One team created a set of requirements for a clone of the IBM BIOS software, and another team, with no overlap, wrote the new BIOS.
Put another way, if i write an article for a magazine (remember those?), on any given topic, and I'm paid for it, I can still write another article on a similar topic for some other magazine, and get paid for it. And I've done exactly that. The results changed dramatically because the audience changed, but the initial topic was the same.
Your knowledge gained during the first project is yours. You've only sold the product of that knowledge.
Jerry Stern https://www.startupware.com
answered Nov 20, 2016 at 01:38 PM Jerry.Stern ♦♦ 16