I have an Approved I-140 from a previous job. My new employer has initiated the PERM. How can I make sure that the attorneys use the priority date from my old I-140?

Even if your lawyers do nothing to use the old priority date, that priority date is your right. The government itself is supposed to run a data sweep. Check their databases and give you the earliest date you are entitled to. If none of the documents reflect that you've been given the earlier priority date it is still not a problem. Hence there is no other procedure required for you to follow. You do not  have to be overly concerned about the attorneys using the old priority date.
 

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