Congrats I485, since now you have passed the I485 step, I guess now you will change your user name in Sajha to GC :)
I don't think there is any clear rule on this but here what I have seen.
One of my friends left the company 2 months after getting the GC, the company was pissed off as he even didn't give two weeks notice (he got a decent offer from Oracle and had to join the project within a week). The president of the company was so mad that he told my friend, he would never give good reference but still he left the company. I think if there was something they (the company) could harm him (report to INS or anything like that), they would do so but nothing happened to him and his family during naturalization process so I think you are safe to move if you really have to. Good Luck.