a lof of lawyer (above 95%) says it's OK to change anytime
Since the new AC21 portability, it's not make sense that you can even change employer before GC but not after GC (according to the bar association). However, make sure you don't sign any agreement with your employer regarding after GC, for example, your employer paid you the cost for GC process and sign a contract with you to ask you saty a certain period of time after your GC approved (lot of company do that), if you break the agreement, your employer may make a complaint to INS for possible fraud. Good luck and consult with your attorney.
Keep in mind that NO any law which mention you should stay a certain period time such as 6 months, the most important is that you do not intent to fraud and keep the intention to join the employer later. Otherwise, there is no difference between 1 week and 6 months.
My laywer is a very conservative one, so if she says OK that's mean something. Also, be sure to stay with the same job description if you can.