I have got PERM certified from current employer. What is best time to change employer now.

My current employer has filed my PERM under EB2 and got certified in Sep-18. Possibly in Feb-18, they will apply for I-140. My Questions are - 1. When can I change the employer and also can retain my Priority Date? 2. If I change it before I-140 approved, will Priority Date be valid for the next employer to apply? 3. My current employer, may not notify me when I140 is approved, How I can get the status of this? 4. Should I wait for my I140 to be approved then change the job? If yes, What are the benefits staying with current employer (on immigration and status)? 5. In case my I140 is approved then I changed the job and my new employer is not applying my GC, will my previous I140 be valid here? If Yes, How long. If No, What is the workaround for this?

Hi. I am also in this same situation. Have you found any solution for this. Could you please update with your situation. Thank you.