I-140 is pending, right time to change employer to retain Priority Date

#1
My I-140 is in pending status applied 4 weeks back.

I got new offer from employer.

I would like to know when is the right time to change employer to carry forward my PD.
Do I need to wait till my I-140 approved with current employer. And once it is approved can I change employer immediately or do I need to wait for 180 days in order to carry forward priority date.

Please advice on this. Thanks
 
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