Yes, you may change your employer. However the new employment must be in the same or similar position. Generally speaking, you are not required to work for your employer, before your green card application is approved. The purpose of the green card application is to allow you to work for the sponsoring (future) employer once your application is approved. Therefore, you may, for example, work on an H-1B Visa for one employer and be sponsored for a green card another, unrelated company.