There's no such rule. The requirement is that the green card applicant must have the intent to work for the sponsoring employer and the sponsoring employer must have the intent to employ the sponsored employee after the employee becomes a green card holder but this isn't inconsistent with finding a great job after getting a green card. This concern typically arises when a green card holder applies to become a citizen but I've never seen this actually be an issue. The concern is even less now with green card portability, which allows green card applicants to change jobs before they receive their green card without jeopardizing their green card applications. The short answer is that it's fine to change jobs soon after getting a green card but still I'd recommend having a quick consultation with an immigration beforehand.