Business conditions change and I can't see what is wrong with you changing positions with your sponsoring employer at any point after each of you have fulfilled your obligations after becoming a PR, i.e. you reported to work and took on the same job your green card was based on, and your employer paid you the specified wage for this. If only a week after this something comes up that is a better opportunity with your employer, just WHO is going to raise an issue with this? It's a win-win for you and your employer and the INS isn't going interfere with conducting normal business practices like this.
Take a somewhat different scenario and you end up getting laid-off a week after you become a PR. Who's going to deny you the opportunity to seek out a new job in that instance? And it wouldn't even be with your sponsoring employer at that point. The consensus on this scenario is that this is OK, so how can an internal move with your employer be less acceptable than this???
Take the new job.