I don't think your case has been assigned. When did you receive the notice? when was the notice issued. If it's very close to the expire date of First FP, then it's not in officer hand. Officer adjudicate cases even if FP expired, only if he's going to approve a case, he will send second FP notice. But that won't be exactly close to the FP expire date. So your case has not been assigned.
At this stage, I'll say wait to change your job. Company has the obligation to send letter to INS informing them you no longer work for the company, worst case can result in your 485 denial, best case will trigger you a RFE. You certainly don't want that, right?