GC is not special!!!回复：如果没有绿卡，雇主可以随时通知你

As far as I know, most of my coworkers are "at will" employee. That means, GC or not, they can fire you anytime, and certainly they can change the terms of your contract any time. You can, of course, choose to not accept and leave the company. This policy has nothing to do with GC or citizenship.