If your company has shifted focus to something you are already familiar with, they should have an advantage in working with you even more.
I read that as the management didn't think of you highly enough, or that they don't think you could pull it off or that what you did is significant enough. They may also have no incentive to dilute their shares for a workable solution by their existing employees (NIH?). Anyway to look at it, it is bad.
In other comments others mentioned non-compete. At least in California non-compete is pretty much unenforcible, but IP right protection is. During your employment they do have IP rights to inventions you made unless (a) they are not in the same field or (b) you have disclosed it as prior invention per a typical employment agreement. Now that since (a) no longer applies, you better make sure you notify them and file a formal amendment to your employment agreement that whatever you worked on is considered prior invention. Or else from now on they may claim the right to not just what you're about to do, but also the hard work you've done. This is serious stuff. People and companies had been destroyed by like this. Either way, I think you can no longer continue your moonlight path on this very subject under their employment (unless you get them to put an exception clause to your employment contract, but I doubt they'll entertain that). I hope you work out something with them so they use your stuff and reward you.