Not legal Advice:
There seems to be some confusion about this. As far as I am aware you can transfer employers at any time after filing an I-485 (as long as you have work authorisation like H1 or EAD for your new job).
The INS can only refuse your I-485 if they take LESS than 180 days to adjudicate it.
In other words you can change jobs at any time after filing and as long as the INS takes more than 180 days to adjudicate your I-485 you are home free regardless of when you actually switched employers.