If your I-485 was pending for at least 180 days, AC21* should mean that working in a different location won't be a problem.
*By the strict letter of the law, AC21 is only for the time while the I-485 is pending. However it would be extremely surprising for a court to have such a strict interpretation that suddenly takes away your freedom to change locations when the GC is approved.
PD: Jan 2003 (EB3 rest of world)
I-485 Approved: July 2007
I am a layman, not a lawyer. What I write here is not official or professional legal advice. In addition, my answers on this forum are specific to the scenarios discussed in each thread and should not be generalized to other situations.