Ah, good question. You will definitely need an immigration attorney to advise how to approach your situation. As soon as you get an educated advice, my ramblings below will become irrelevant
OK, here goes.
Since we are talking about an "Outstanding Researcher" subcategory of EB-1, you still need a "permanent" job offer from the sponsoring employer who has filed I-140. You don't have a Labor Certification, though.
So, since you have I-140 and a requirement of a job offer for a permanent position, my best guess that AC21 actually applies to EB-1 OR. However some sources indicate that the job offer may not necessarily be that "permanent", but I-140 filing description clearly states otherwise.
Furthermore, since you don't have a Labor Certification, there is less chance of running into problems after the job change; the problems related to geographical location, job title and description and other stuff which usually travels along with EB-3.
Well, if you are really have to change the job
right now, then go ahead and change it. Get a new job offer with the same title and job description resembling the original offer. The salary, lacking Labor Certification guidance, is not supposed to be less than the original job offer.
If you can wait, then keep the current job until the I-485 is adjudicated. Existing AC21 regulations are pretty vague and, as you have already noticed, do not mention any EB-1 OR specifics. On the other hand, you have less to lose if your I-485 will be turned down, so, I guess, it is a simple judgement call - how much risk you are willing to take.
