As I think logically, The permanent residency is for future employment. Hence, it does matter with who you work for. All that is expected is that, you have intention to work for the company X that filed your labor and I-140.
Also I thought of sharing my chain of thoughts while thinking through this scenario, since you have an approved I-140, you may still file I-485 if your PD becomes current. Should you get your I-485 approved withing 6 months, you're expected to work for the company that filed your Labor/I-140 petition. Should your I-485 pass 180 days, and you do not want to tie yourself to the company that filed Labor/I-140, you may claim refuge under the provisions of AC-21.
In both cases above, you may or may not get an interview letter. So be prepared either case.
However, It would be prudent on your part to consult an Experienced Immigration attorney to get the clouds of doubts cleared.
Gurus, Share your views/findings on this thread.