I am a silent reader in these forums, but now, I need to throw my 2 cents in.
I guess it does not matter when exactly you leave your employer after you get your green card.
The law does not state that you need to work six months or a year with an employer after getting the green card. The juridical term of permanent employment concerns an undefined period, after which one side decides to break the deal.
Now let us consider the side of the employee. It seems that if you have a solid enough reason for leaving that company, it may work out. You are working for many years already with that employer, they treated you like crap not giving you any raise or small raises, gave you no professional promotion, and they discriminated you, or harassed you.
In the light of the US laws, these are enough reasons to leave an employer, no matter when. The only catch concerning USCIS is to have it very well documented...
Let me tell you my specific situation. I work for the employer X for almost four years. They treated me like crap, in four years I got only two petty raises. Since I applied for a green card, I got further skills. I graduated with honors an MBA and got PMI certification in project management. Do you think my employer gave a penny for these skills? No, they did not care and did not offer any recognition or promotion of any way, keeping me to the same stupid programming position I held for years.
In your vision, is this a solid reason to leave? I have documentation proving my employment history and lack of professional recognition. I had already an offer as project manager, significantly higher than what I got now, and I needed to turn it down because of stupid green card situation.
I guess I proved more than enough the intention of working in permanent position with this employer working for four years in the same stupid position and the same stupid salary. Only USCIS and DOL screwed me up with this wait, because I should have had my green card long ago (a year and half ago) and should not be in this situation in the first place.
My conclusion is that leaving an employer after green card is a case-to-case matter. If you never worked for the sponsoring employer, joined, and left shortly after green card, even six months, this I think is an intention of fraud.
However, if you worked in permanent position years before getting the friggin' green card and you are compelled to leave because of some special reason, then better consult an attorney to see if your reason is strong enough to resist the attempt of rescission for your green card...
Have fun...