You cannot change your job without getting your I-140 approval. Its true that you will get EAD based on your I-485 application. Also, its not 6 months, but after 3 months of your I-485 filing from a local INS office provided you didn't recieve the same from the INS center where you filed it (this is because some of the centers are processing EADs sooner than 3 months).
If you lose your job (it doesn't matter if you have a EAD or not) before getting a I-140 approval, the underlying petitions (485, EAD, AP) will get denied automatically.
Good Luck.