If that's the case, I may have to ask the company to let me stay in the city or work from home,
-Fine if they agree.
or have to delay Fragomen starting the AC21 process.
-It doesn't matter.
140 approved + 485 pending <180 days, relocation with same employer = change employer, both have very high risk if USCIS process the case before 180 days.
-Don't think it is high risky in either of the situation. If you have a job no matter whether you work for the same emplyer at different location or for different employer, USCIS normally will not deny your case even if the I-485 pending is < 180 days though technically they may, but practically they probably won't. Also, based on AC21, there is not specific saying that USCIS will deny your I-485 in this case. So, I think it is a gray area (not a absolute rule) and thus, USCIS normally is not very strick. But if the I-140 sponsor withdraw the I-140, your I-485 will be void.
However, I still suggest you to stay with your current employer even if you will work at a different location in case they withdraw the I-140 (you need to prepare for the worst situation).