totally agree with the above, but I'd like to point out that any contract is an agreement between two parties. If the two parties involved decide to change that agreement amicably, that's fine. Court battles are the last resort, not the first move.

So even if the legal opinion is that you don't have a leg to stand on, you could still get everything you want if your employer agrees to it. I would try to negotiate from that stance, instead of a legal one. So "yes, boss, I understand that contractually the IP is yours, but the community is mine, and the goodwill is mine, and the IP is worth nothing without those, because the community will just fork the code. So let's talk about how this is going to work..."